[MachineSink] Some more preserving of debug location when rematerialising an...

 [MachineSink] Some more preserving of debug location when rematerialising an instruction to replace a COPY (#73155)

Somewhat similar to ef9bcace
([MachineSink][AArch64] Preserve debug location when rematerialising
an instruction to replace a COPY (#72685))
reuse the debug location of the COPY, iff the rematerialised instruction
did not have a location.

Fixes a regression in `DebugInfo/AArch64/constant-dbgloc.ll` after
enabling sink-and-fold.