[llvm] [AMDGPU] Extend promotion of alloca to vectors (PR #127973)

[Carl Ritson via llvm-commits]

perlfu wrote:

Note: I have changed the max vector size to be based on number of 32b registers.  With the current value of 16, this mean some alloca which were previously promoted are no longer promoted.
See promote-alloca-subvecs.ll and vector-alloca-limits.ll where I had to increase the limit for the tests to pass.
Alternatively we could raise the default to 32 to allow 16 x i64/double/ptr to be promoted as previously, with the effect that 32 x i32/float would now be promoted.

https://github.com/llvm/llvm-project/pull/127973