AMDGPU: Look through a bitcast user of an out argument

This allows handling of a lot more of the interesting
cases in Blender. Most of the large functions unlikely
to be inlined have this pattern.

This is a special case for what clang emits for OpenCL 3
element vectors. Annoyingly, these are emitted as
<3 x elt>* pointers, but accessed as <4 x elt>* operations.
This also needs to handle cases where a struct containing
a single vector is used.

llvm-svn: 309419