[RISCV] Adjust sdloc when creating an extend for widening instructions (#82040)

We were using the SDLoc corresponding to the original arithmetic
instruction, but here using the SDLoc corresponding to the original
extend if we need to introduce a new narrower extend seems cleaner.

As can be seen in the test diffs, this very minorly impacts scheduling
and register allocation by given the scheduler a hint from original
program order.