cmd/compile: refactor some code in loopbce.go

This CL extracts the logic for pattern-matching an induction
variable into a separate function, in preparation for next CL
where we would need to call it multiple times.

No functional changes, passes toolstash -cmp.

Change-Id: Ic52391e6c1b2e72bae32a0f3f65dfea321caaf4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/195737


Reviewed-by: David Chase <drchase@google.com>