[CodeGen] Correctly handle non-standard cases in RemoveLoadsIntoFakeUses (#111551)

In the RemoveLoadsIntoFakeUses pass, we try to remove loads that are
only used by fake uses, as well as the fake u

[CodeGen] Correctly handle non-standard cases in RemoveLoadsIntoFakeUses (#111551)

In the RemoveLoadsIntoFakeUses pass, we try to remove loads that are
only used by fake uses, as well as the fake use in question. There are
two existing errors with the pass however: it incorrectly examines every
operand of each FAKE_USE, when only the first is relevant (extra
operands will just be "killed" regs assigned by a previous pass), and it
ignores cases where the FAKE_USE register is not an exact match for the
loaded register, which is incorrect as regalloc may choose to load a
wider value than the FAKE_USE required pre-regalloc. This patch fixes
both of these cases.

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[LLVM] Add HasFakeUses to MachineFunction (#110097)

Following the addition of the llvm.fake.use intrinsic and corresponding
MIR instruction, two further changes are planned: to add an
-fextend-lif

[LLVM] Add HasFakeUses to MachineFunction (#110097)

Following the addition of the llvm.fake.use intrinsic and corresponding
MIR instruction, two further changes are planned: to add an
-fextend-lifetimes flag to Clang that emits these intrinsics, and to
have -Og enable this flag by default. Currently, some logic for handling
fake uses is gated by the optdebug attribute, which is intended to be
switched on by -fextend-lifetimes (and by extension -Og later on).
However, the decision was made that a general optdebug attribute should
be incompatible with other opt_ attributes (e.g. optsize, optnone),
since they all express different intents for how to optimize the
program. We would still like to allow -fextend-lifetimes with optsize
however (i.e. -Os -fextend-lifetimes should be legal), since it may be a
useful configuration and there is no technical reason to not allow it.

This patch resolves this by tracking MachineFunctions that have fake
uses, allowing us to run passes that interact with them and skip passes
that clash with them.

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[ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (#86149)

This patch is part of a set of patches that add an `-fextend-lifetimes`
flag to clang, which extends the lifetimes of

[ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (#86149)

This patch is part of a set of patches that add an `-fextend-lifetimes`
flag to clang, which extends the lifetimes of local variables and
parameters for improved debuggability. In addition to that flag, the
patch series adds a pragma to selectively disable `-fextend-lifetimes`,
and an `-fextend-this-ptr` flag which functions as `-fextend-lifetimes`
for this pointers only. All changes and tests in these patches were
written by Wolfgang Pieb (@wolfy1961), while Stephen Tozer (@SLTozer)
has handled review and merging. The extend lifetimes flag is intended to
eventually be set on by `-Og`, as discussed in the RFC
here:

https://discourse.llvm.org/t/rfc-redefine-og-o1-and-add-a-new-level-of-og/72850

This patch implements a new intrinsic instruction in LLVM,
`llvm.fake.use` in IR and `FAKE_USE` in MIR, that takes a single operand
and has no effect other than "using" its operand, to ensure that its
operand remains live until after the fake use. This patch does not emit
fake uses anywhere; the next patch in this sequence causes them to be
emitted from the clang frontend, such that for each variable (or this) a
fake.use operand is inserted at the end of that variable's scope, using
that variable's value. This patch covers everything post-frontend, which
is largely just the basic plumbing for a new intrinsic/instruction,
along with a few steps to preserve the fake uses through optimizations
(such as moving them ahead of a tail call or translating them through
SROA).

Co-authored-by: Stephen Tozer <stephen.tozer@sony.com>

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