cmd/compile: constant-fold switches early in compilation

cmd/compile: constant-fold switches early in compilation

So that the inliner knows all the other cases are dead and doesn't
accumulate any cost for them.

The canonical case for this is switching on runtime.GOOS, which occurs
several places in the stdlib.

Fixes #50253

Change-Id: I44823aaebb6c1b03c9b0c12d10086db81954350f
Reviewed-on: https://go-review.googlesource.com/c/go/+/399694
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>