AMDGPU: Add offsets to MMO when lowering buffer intrinsics

Summary:
Without offsets on the MachineMemOperands (MMOs),
MachineInstr::mayAlias() will return true for all reads and writes to the
same resource descriptor.  This leads to O(N^2) complexity in the MachineScheduler
when analyzing dependencies of buffer loads and stores.  It also limits
the SILoadStoreOptimizer from merging more instructions.

This patch reduces the compile time of one pathological compute shader
from 12 seconds to 1 second.

Reviewers: arsenm, nhaehnle

Reviewed By: arsenm

Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, t-tye, hiraditya, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65097

llvm-svn: 374087