Together with leading known-zero bit tracking for getelementptr, this
recovers most of the code quality that was lost for AMDGPU's SI subtarget
with a previous commit.
AMDGPU has LDS (local) and GDS (region) address spaces that use 32 bit
pointers, even though the underlying memory is only 64 kiB, and therefore
pointers to global values always fit into 16 bits.
This recovers a lot of the code quality lost in "AMDGPU: Restrict DS
load/store vectorizing on SI".