cmd/compile: rewrite a, b = f() to use temporaries when type not identical

If any of the LHS expressions of an OAS2FUNC are not identical to the
respective function call results, escape analysis mishandles the
implicit conversion, causes memory corruption.

Instead, we should insert autotmps like we already do for f(g()) calls
and return g() statements.

Fixes #46725

Change-Id: I71a08da0bf1a03d09a023da5b6f78fb37a4a4690
Reviewed-on: https://go-review.googlesource.com/c/go/+/327651


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