[ARM] Fix Crashes in fp16/bf16 Inline Asm

We were still seeing occasional crashes with inline assembly blocks
using fp16/bf16 after my previous patches:
- https://reviews.llvm.org/rGff4027d152d0
- h

[ARM] Fix Crashes in fp16/bf16 Inline Asm

We were still seeing occasional crashes with inline assembly blocks
using fp16/bf16 after my previous patches:
- https://reviews.llvm.org/rGff4027d152d0
- https://reviews.llvm.org/rG7d15212b8c0c
- https://reviews.llvm.org/rG20b2d11896d9

It turns out:
- The original two commits were wrong, and we should have always been
choosing the SPR register class, not the HPR register class, so that
LLVM's SelectionDAGBuilder correctly did the right splits/joins.
- The `splitValueIntoRegisterParts`/`joinRegisterPartsIntoValue` changes
from rG20b2d11896d9 are still correct, even though they sometimes
result in inefficient codegen of casts between fp16/bf16 and i32/f32
(which is visible in these tests).

This patch fixes crashes in `getCopyToParts` and when trying to select
`(bf16 (bitconvert (fp16 ...)))` dags when Neon is enabled.

This patch also adds support for passing fp16/bf16 values using the 'x'
constraint that is LLVM-specific. This should broadly match how we pass
with 't' and 'w', but with a different set of valid S registers.

Differential Revision: https://reviews.llvm.org/D147715

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[ARM] Fix Crash in 't'/'w' handling without fp16/bf16

After https://reviews.llvm.org/rGff4027d152d0 and
https://reviews.llvm.org/rG7d15212b8c0c we saw crashes in SelectionDAG
when trying to use thes

[ARM] Fix Crash in 't'/'w' handling without fp16/bf16

After https://reviews.llvm.org/rGff4027d152d0 and
https://reviews.llvm.org/rG7d15212b8c0c we saw crashes in SelectionDAG
when trying to use these constraints when you don't have the fp16 or
bf16 extensions.

However, it is still possible to move 16-bit floating point values into
the right place in S registers with a normal `vmov`, even if we don't
have fp16 instructions we can use within the inline assembly string.
This patch therefore fixes the crash.

I think the reason we weren't getting this crash before is because I
think the __fp16 and __bf16 types got an error diagnostic in the Clang
frontend when you didn't have the right architectural extensions to use
them. This restriction was recently relaxed.

The approach for bf16 needs a bit more explanation. Exactly how BF16 is
legalized was changed in rGb769eb02b526e3966847351e15d283514c2ec767 -
effectively, whether you have the right instructions to get a bf16 value
into/out of a S register with MoveTo/FromHPR depends on hasFullFP16, but
whether you use a HPR for a value of type MVT::bf16 depends on hasBF16.
This is why the tests are not changed by `+bf16` vs `-bf16`, but I've
left both sets of RUN lines in case this changes in the future.

Test Changes:
- Added more testing for testing inline asm (the core part)
- fp16-promote.ll and pr47454.ll show improvements where unnecessary
fp16-fp32 up/down-casts are no longer emitted. This results in fewer
libcalls where those casts would be done with a libcall.
- aes-erratum-fix.ll is fairly noisy, and I need to revisit this test so
that the IR is more minimal than it is right now, because most of the
changes in this commit do not relate to what AES is actually trying to
verify.

Differential Revision: https://reviews.llvm.org/D143711

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[ARM] Support fp16/bf16 using w constraint

fp16 and bf16 values can be used in GCC's inline assembly using the "w"
constraint, which means "VFP floating-point registers d0-d31" - fp16 and
bf16 value

[ARM] Support fp16/bf16 using w constraint

fp16 and bf16 values can be used in GCC's inline assembly using the "w"
constraint, which means "VFP floating-point registers d0-d31" - fp16 and
bf16 values are stored in S registers (which alias the D registers).

This change ensures that LLVM is compatible with GCC for programs that
use fp16 and the 'w' constraint.

Differential Revision: https://reviews.llvm.org/D135662

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[ARM] Support fp16/bf16 using t constraint

fp16 and bf16 values can be used in GCC's inline assembly using the "t"
constraint, which means "VFP floating-point registers s0-s31" - fp16 and
bf16 value

[ARM] Support fp16/bf16 using t constraint

fp16 and bf16 values can be used in GCC's inline assembly using the "t"
constraint, which means "VFP floating-point registers s0-s31" - fp16 and
bf16 values are stored in S registers too.

This change ensures that LLVM is compatible with GCC for programs that
use fp16 and the 't' constraint.

Fixes #57753

Differential Revision: https://reviews.llvm.org/D134553

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