cmd/compile: Unify & improve struct comparisons
Partially fixes https://github.com/golang/go/issues/38674
The first commit has the actual unification, the second commit just cleans things up by moving shared code into its own package for clarity.
Change-Id: I85067f8b247df02f94684ec1297a1a42263bba0c
GitHub-Last-Rev:
370a4ecad315f945b62195f8daddca693345a0c7
GitHub-Pull-Request: golang/go#52315
Reviewed-on: https://go-review.googlesource.com/c/go/+/399542
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>