cmd/compile: simplify a few bits of the code

cmd/compile: simplify a few bits of the code

Remove an unused type, a few redundant returns and replace a few slice
append loops with a single append.

Change-Id: If07248180bae5631b5b152c6051d9635889997d5
Reviewed-on: https://go-review.googlesource.com/66851
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>