AMDGPU: Do not bitcast atomic load in IR (#90060)

These hooks should be removed. This is a trivial legalization transform
the legalizer needs to support. The IR just complicates things, and it
was losing metadata. Implement the DAG promotion support, and switch
AMDGPU over to using it.

Really we'd be a lot better off merging ATOMIC_LOAD and LOAD like
GlobalISel does.