[X86] Avoid SFB - Fix inconsistent codegen with/without debug info(2)

The function findPotentialBlockers may consider debug info instructions as
potential blockers and may stop searching for a store-load pair prematurely.

This patch corrects this and tests the cases where the store is separated
from the load by more than InspectionLimit debug instructions.

Patch by Chris Dawson.

Differential Revision: https://reviews.llvm.org/D62408

llvm-svn: 364829