[MachineLICM] Debug intrinsics shouldn't affect hoist decisions

Summary:
When checking if an instruction stores to a given frame index, check
that the instruction can write to memory before looking at the memory
operands list to avoid e.g. DBG_VALUE instructions that reference a
frame index preventing a load from that index from being hoisted.

Reviewers: dblaikie, MatzeB, qcolombet, reames, javed.absar

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D46284

llvm-svn: 331549