[MachineCopyPropagation] Fix differences in code gen when compiling with -g

[MachineCopyPropagation] Fix differences in code gen when compiling with -g

Fixes bugs [[ https://bugs.llvm.org/show_bug.cgi?id=50580 | 50580 ]] and [[ https://bugs.llvm.org/show_bug.cgi?id=49446 | 49446  ]]

When compiling with -g "DBG_VALUE <reg>"  instructions are added in the MIR, if such a instruction is inserted between instructions that use <reg> then MachineCopyPropagation invalidates <reg> , this causes some copies  to not be propagated and causes differences in code generation (ex bugs 50580 and 49446 ).  DBG_VALUE instructions should be ignored  since they don't actually modify the register.

Reviewed By: lkail

Differential Revision: https://reviews.llvm.org/D104394