Somewhat similar to ef9bcace834e63f25bbbc5e8e2b615f89d85fb2f ([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.