cmd/compile: when merging instructions, prefer line number of faulting insn
Normally this happens when combining a sign extension and a load. We
want the resulting combo-instruction to get the line number of the
load, not the line number of the sign extension.
For each rule, compute where we should get its line number by finding
a value on the match side that can fault. Use that line number for
all the new values created on the right-hand side.
Fixes #27201
Change-Id: I19b3c6f468fff1a3c0bfbce2d6581828557064a3
Reviewed-on: https://go-review.googlesource.com/c/156937 Reviewed-by: David Chase <drchase@google.com>