cmd/compile: add astdump debug flag

cmd/compile: add astdump debug flag

This was extraordinarily useful for inlining work.
I have cleaned it up somewhat, and did some additional tweaks
after working on changes to bloop.

-gcflags=-d=astdump=SomeFunc
-gcflags=-d=astdump=SomeSubPkg.SomeFunc
-gcflags=-d=astdump=Some/Pkg.SomeFunc
-gcflags=-d=astdump=~YourRegExpHere

Change-Id: I3f98601ca96c87d6b191d4b64b264cd236e6d8bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/629775
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>