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

Fixes https://bugs.llvm.org/show_bug.cgi?id=40969

The functions findPotentiallyBlockedCopies and buildCopy are currently not
accounting for the presence of debug instructions. In the former this results
in the optimization not being trigerred, and in the latter results in
inconsistent codegen.

This patch enables the optimization to be performed in a debug build and
ensures the codegen is consistent with non-debug builds.

Patch by Chris Dawson.

Differential Revision: https://reviews.llvm.org/D61680

llvm-svn: 360436