During post RA scheduling, do not try to chase reg defs. to preserve DBG_VALUEs....

During post RA scheduling, do not try to chase reg defs. to preserve DBG_VALUEs. This approach has several downsides, for example, it does not work when dbg value is a constant integer, it does not work if reg is defined more than once, it places end of debug value range markers in the wrong place. It even causes misleading incorrect debug info when duplicate DBG_VALUE instructions point to same reg def.

Instead, use simpler approach and let DBG_VALUE follow its predecessor instruction. After live debug value analysis pass, all DBG_VALUE instruction are placed at the right place. Thanks Jakob for the hint!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132483 91177308-0d34-0410-b5e6-96231b3b80d8