cmd/compile: call missing popdcl in various genxxx functions

Not calling popdcl doesn't have an impact on generated code but
the result is a growing (rather than empty) stack of symbols,
possibly causing more data to remain alive than necessary.

Also: minor cleanups.

Change-Id: Ic4fdbcd8843637d69ab1aa15e896a7e6339bc990
Reviewed-on: https://go-review.googlesource.com/20554


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