AMDGPU/SI: Remove hack for selecting < 32-bit loads to MUBUF instructions

Summary:
The isGlobalLoad() query was returning true for constant address space loads
with memory types less than 32-bits, which is wrong.  This logic has been
replaced with PatFrag in the TableGen files, to provide the same functionality.

Reviewers: arsenm

Subscribers: arsenm, kzhuravl, llvm-commits

Differential Revision: http://reviews.llvm.org/D21696

llvm-svn: 274521