cmd/compile: fix reassignVisitor

reassignVisitor was short-circuiting on assignment statements after
checking the LHS, but there might be further assignment statements
nested within the RHS expressions.

Fixes #42284.

Change-Id: I175eef87513b973ed5ebe6a6527adb9766dde6cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/266618


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