[AMDGPU] Fix for negative offsets in buffer/tbuffer intrinsics
The new buffer/tbuffer intrinsics handle an out-of-range immediate
offset by moving/adding offset&-4096 to a vgpr, leaving an in-range
immediate offset, with a chance of the move/add being CSEd for similar
However it turns out that a negative offset in a vgpr is illegal, even
if adding the immediate offset makes it legal again.
Therefore, this commit disables the offset&-4096 thing if the offset is
Differential Revision: https://reviews.llvm.org/D52683