cmd/compile: replace isSigned(t) with t.IsSigned()

cmd/compile: replace isSigned(t) with t.IsSigned()

No change in semantics, just removing an unneeded helper.

Also align rules a bit.

Change-Id: Ie4dabb99392315a7700c645b3d0931eb8766a5fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/483439
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>