[SystemZ] Handle DBG_VALUE instructions in two places in backend.

[SystemZ]  Handle DBG_VALUE instructions in two places in backend.

Two backend optimizations failed to handle cases when compiled with -g, due
to failing to consider DBG_VALUE instructions. This was in
SystemZTargetLowering::emitSelect() and
SystemZElimCompare::getRegReferences().

This patch makes sure that DBG_VALUEs are recognized so that they do not
affect these optimizations.

Tests for branch-on-count, load-and-trap and consecutive selects.

Review: Ulrich Weigand
https://reviews.llvm.org/D57048

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351928 91177308-0d34-0410-b5e6-96231b3b80d8