[AMDGPU] Scalarization of global uniform loads.

Summary:
LC can currently select scalar load for uniform memory access
basing on readonly memory address space only. This restriction
originated from the fact that in HW prior to VI vector and scalar caches
are not coherent. With MemoryDependenceAnalysis we can check that the
memory location corresponding to the memory operand of the LOAD is not
clobbered along the all paths from the function entry.

Reviewers: rampitec, tstellarAMD, arsenm

Subscribers: wdng, arsenm, nhaehnle

Differential Revision: https://reviews.llvm.org/D26917

llvm-svn: 289076