[AMDGPURewriteOutArguments] Don't use pointer element type

Instead of using the pointer element type, look at how the pointer
is actually being used in store instructions, while looking through
bitcasts. This makes the transform compatible with opaque pointers
and a bit more general.

It's worth noting that I have dropped the 3-vector to 4-vector
shufflevector special case, because this is now handled in a
different way: If the value is actually used as a 4-vector, then
we're directly going to use that type, instead of shuffling to a
3-vector in between.

Differential Revision: https://reviews.llvm.org/D119237