[RISCV] Enable store clustering by default (#73796)

Builds on #73789, enabling store clustering by default using the same
heuristic.

[RISCV] Re-separate unaligned scalar and vector memory features in the backend. (#88954)

This is largely a revert of commit
e81796671890b59c110f8e41adc7ca26f8484d20.

As #88029 shows, there exist

[RISCV] Re-separate unaligned scalar and vector memory features in the backend. (#88954)

This is largely a revert of commit
e81796671890b59c110f8e41adc7ca26f8484d20.

As #88029 shows, there exists hardware that only supports unaligned
scalar.

I'm leaving how this gets exposed to the clang interface to a future
patch.

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[RISCV] Collapse fast unaligned access into a single feature [nfc-ish] (#73971)

When we'd originally added unaligned-scalar-mem and
unaligned-vector-mem, they were separated into two parts under th

[RISCV] Collapse fast unaligned access into a single feature [nfc-ish] (#73971)

When we'd originally added unaligned-scalar-mem and
unaligned-vector-mem, they were separated into two parts under the
theory that some processor might implement one, but not the other. At
the moment, we don't have evidence of such a processor. The C/C++ level
interface, and the clang driver command lines have settled on a single
unaligned flag which indicates both scalar and vector support unaligned.
Given that, let's remove the test matrix complexity for a set of
configurations which don't appear useful.

Given these are internal feature names, I don't think we need to provide
any forward compatibility. Anyone disagree?

Note: The immediate trigger for this patch was finding another case
where the unaligned-vector-mem wasn't being properly serialized to IR
from clang which resulted in problems reproducing assembly from clang's
-emit-llvm feature. Instead of fixing this, I decided getting rid of the
complexity was the better approach.

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[RISCV] Set Fast flag for unaligned memory accesses

The +unaligned-scalar-mem and +unaligned-vector-mem features were added in
D126085 and D149375 respectively to allow subtargets to indicate that
t

[RISCV] Set Fast flag for unaligned memory accesses

The +unaligned-scalar-mem and +unaligned-vector-mem features were added in
D126085 and D149375 respectively to allow subtargets to indicate that
they supported misaligned loads/stores with "sufficient" performance.
This is separate from whether or not the target actually supports
misaligned accesses, which could be determined from Zicclsm.

This patch enables the Fast flag under the assumption that any subtarget
that declares support for +unaligned-*-mem will want to opt into
optimisations that take advantage of misaligned scalar accesses, such as
store merging.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D150771

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[RISCV] Expand memset.inline test coverage [nfc]

Add coverage for unaligned overlap cases, and for vector stores.

Note that the vector memset here is coming from store combining, not memset lowerin

[RISCV] Expand memset.inline test coverage [nfc]

Add coverage for unaligned overlap cases, and for vector stores.

Note that the vector memset here is coming from store combining, not memset lowering.

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[RISCV] Add memset.inline test coverage with and without V [nfc]