[AMDGPU] Vectorize alloca thru bitcast

This is mostly useful if alloca element type is not integer
and then casted to an integer for load or store. We now can
vectorize an [i32] alloca but cannot do so for [float].

There also a separate patch needed to properly lower 64 bit
types after they vectorized. At the moment these are lowered
via scratch anyway.

Differential Revision: https://reviews.llvm.org/D79641