[AMDGPU] Use new target MMO flag MONoClobber

[AMDGPU] Use new target MMO flag MONoClobber

This allows us to set the noclobber flag on (the MMO of) a load
instruction instead of on the pointer. This fixes a bug where noclobber
was being applied to all loads from the same pointer, even if some of
them were clobbered.

Differential Revision: https://reviews.llvm.org/D118775