[GlobalISel][DebugInfo] Salvage trivially dead instructions

Use salvageDebugInfo for instructions erased as trivially dead in
GlobalISel.

It would be helpful to implement support of G_PTR_ADD and G_FRAME_INDEX
in salvageDebugInfo in future in order to preserve more variable
location.

Reviewed by: arsenm

Differential Revision: https://reviews.llvm.org/D133986