[CodeGen] Fix bugs in LiveDebugVariables when debug labels are generated.

Remove DBG_LABELs in LiveDebugVariables and generate them in
VirtRegRewriter.

This bug is reported in
https://bugs.chromium.org/p/chromium/issues/detail?id=898152.

Differential Revision: https://reviews.llvm.org/D54465

llvm-svn: 351525