Update LiveDebugVariables after live range splitting.

Update LiveDebugVariables after live range splitting.

After a virtual register is split, update any debug user variables that resided
in the old register. This ensures that the LiveDebugVariables are still correct
after register allocation.

This may create DBG_VALUE instructions that place a user variable in a register
in parts of the function and in a stack slot in other parts. DwarfDebug
currently doesn't support that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130998 91177308-0d34-0410-b5e6-96231b3b80d8