[AMDGPU] Fix for negative offsets in buffer/tbuffer intrinsics

Summary:
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
loads/stores.

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
negative.

Differential Revision: https://reviews.llvm.org/D52683

Change-Id: Ie02f0a74f240a138dc2a29d17cfbd9e350e4ed13
llvm-svn: 343672