[LowerMemIntrinsics] Use i8 GEPs in memcpy/memmove lowering (#112707)

The IR lowering of memcpy/memmove intrinsics uses a target-specific type
for its load/store operations. So far, the loaded and

[LowerMemIntrinsics] Use i8 GEPs in memcpy/memmove lowering (#112707)

The IR lowering of memcpy/memmove intrinsics uses a target-specific type
for its load/store operations. So far, the loaded and stored addresses
are computed with GEPs based on this type. That is wrong if the
allocation size of the type differs from its store size: The width of
the accesses is determined by the store size, while the GEP stride is
determined by the allocation size. If the allocation size is greater
than the store size, some bytes are not copied/moved.

This patch changes the GEPs to use i8 addressing, with offsets based on
the type's store size. The correctness of the lowering therefore no
longer depends on the type's allocation size.

This is in support of PR #112332, which allows adjusting the memcpy loop
lowering type through a command line argument in the AMDGPU backend.

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[AMDGPU] Allow casts between the Global and Constant Addr Spaces in isValidAddrSpaceCast (#112493)

So far, isValidAddrSpaceCast only allows casts to the flat address
space and between the constant(

[AMDGPU] Allow casts between the Global and Constant Addr Spaces in isValidAddrSpaceCast (#112493)

So far, isValidAddrSpaceCast only allows casts to the flat address
space and between the constant(32) address spaces. It does not allow
casting between the global and constant address spaces, even though they
alias. That affects, e.g., the lowering of memmoves from the constant to
the global address space in LowerMemIntrinsics, since that requires
aliasing address spaces to be castable.

This patch relaxes isValidAddrSpaceCast and allows such casts. It also
includes a memmove test that would crash with the previous
implementation because the memmove IR lowering would not be
applicable for the move from constant AS to global AS.

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