cmd/compile: lay out exit post-dominated blocks at the end

Complete a long-standing TODO in the code.

Exit blocks are cold code, so we lay them out at the end of the function.
Blocks that are post-dominated by exit blocks are also ipso facto exit blocks.
Treat them as such.

Implement using a simple loop, because there are generally very few exit blocks.

In addition to improved instruction cache, this empirically yields
better register allocation.

Binary size impact:

file    before    after     Δ       %       
cgo     4812872   4808776   -4096   -0.085% 
fix     3370072   3365976   -4096   -0.122% 
vet     8252280   8248184   -4096   -0.050% 
total   115052984 115040696 -12288  -0.011% 

This also appears to improve compiler performance
(-0.15% geomean time/op, -1.20% geomean user time/op),
but that could just be alignment effects.
Compiler benchmarking hasn't been super reliably recently,
and there's no particular reason to think this should
speed up the compiler that much.

Change-Id: I3d262c4f5cb80626a67a5c17285e2fa09f423c00
Reviewed-on: https://go-review.googlesource.com/c/go/+/227217


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>