[CodeGen] Avoid handling DBG_VALUE in the LivePhysRegs (addUses,removeDefs,stepForward)

Summary:
This patch prevents DBG_VALUE instructions from influencing
LivePhysRegs::stepBackwards and stepForwards.  In at least one case,
specifically branch folding, the stepBackwards logic was having an
influence on code generation.  The result was that certain code
compiled with '-g -O2' would differ from that compiled with '-O2'
alone. It seems that the original logic, accounting for DBG_VALUE,
was influencing the placement of an IMPLICIT_DEF which had a later
impact on how blocks were processed in branch folding.

Reviewers: kparzysz, MatzeB

Reviewed By: kparzysz

Subscribers: bjope, llvm-commits

Tags: #debug-info

Differential Revision: https://reviews.llvm.org/D43850

llvm-svn: 327862