[LiveDebugValues] Tracking copying value between registers

During the execution of long functions or functions that have a lot of
inlined code it could come to the situation where tracked value could be
transferred from one register to another. The transfer is recognized only if
destination register is a callee saved register and if source register is
killed. We do not salvage caller-saved registers since there is a great
chance that killed register would outlive it.

Patch by Nikola Prica.

Differential Revision: https://reviews.llvm.org/D44016

llvm-svn: 336978