[AMDGPU] Fix deleting of move-immediate instructions after folding

SIInstrInfo::FoldImmediate tried to delete move-immediate instructions
after folding them into their only use. This did not work because it was
checking hasOneNonDBGUse after doing the fold, at which point there
should be no uses. This seems to have no effect on codegen, it just
means less stuff for DCE to clean up later.

Differential Revision: https://reviews.llvm.org/D120815