Rewrite and simplify the rewriteMOV function in preparation for eliminating it
entirely. Improve some error messages in the process.
Change-Id: Id9a77be5174d46cc23651930c2e9068ee6555690
Reviewed-on: https://go-review.googlesource.com/c/go/+/344458
Trust: Joel Sing <joel@sing.id.au> Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>