cmd/compile: more Isfoo Type cleanups

cmd/compile: more Isfoo Type cleanups

Replace isideal(t) with t.IsUntyped().
Replace Istype(t, k) with t.IsKind(k).
Replace isnilinter(t) with t.IsEmptyInterface().

Also replace a lot of t.IsKind(TFOO) with t.IsFoo().

Replacements prepared mechanically with gofmt -w -r.

Passes toolstash -cmp.

Change-Id: Iba48058f3cc863e15af14277b5ff5e729e67e043
Reviewed-on: https://go-review.googlesource.com/21424
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>