[dev.typeparams] cmd/compile: don't compile blank functions

After typechecking a blank function, we can clear out its body and
skip applying middle-end optimizations (inlining, escape analysis). We
already skip sending them through SSA, and the previous CL updated
inlining and escape analysis regress tests to not depend on compiling
blank functions.

Updates #47446.

Change-Id: Ie678763b0e6ff13dd606ce14906b1ccf1bbccaae
Reviewed-on: https://go-review.googlesource.com/c/go/+/338095


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>