[AMDGPU] Extend promote alloca vectorization

Promote alloca can vectorize a small array by bitcasting it to a
vector type. Extend vectorization for the case when alloca is
already a vector type. We still want to replace GEPs with an
insert/extract element instructions in this case.

Differential Revision: https://reviews.llvm.org/D54219

llvm-svn: 346376