Skip over prefetches

Summary: Skip over prefetches when assigning debug info to instructions with memory operands. This way, the debug info is stable after instrumenting a binary with prefetches, allowing for iterative profiling and instrumentation.

Reviewers: davidxl

Reviewed By: davidxl

Subscribers: aprantl, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61789

llvm-svn: 360471