[dev.regabi] cmd/compile: simplify and optimize reorder3

reorder3 is the code responsible for ensuring that evaluation of an
N:N parallel assignment statement respects the order of evaluation
rules specified for Go.

This CL simplifies the code and improves it from an O(N^2) algorithm
to O(N).

Passes toolstash -cmp.

Change-Id: I04cd31613af6924f637b042be8ad039ec6a924c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/280437


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>