[globalisel][legalizer] Fix DebugLoc bugs caught by a prototype lost-location verifier

The legalizer has a tendency to lose DebugLoc's when expanding or
combining instructions. The verifier that detected these isn't ready
for upstreaming yet but this patch fixes the cases that came up when
applying it to our out-of-tree backend's CodeGen tests.

This pattern comes up a few more times in this file and probably in
the backends too but I'd prefer to fix the others separately (and
preferably when the lost-location verifier detects them).