cmd/compile: use t.IsFoo() instead of Isfoo[t.Etype]

This allows us to get rid of Isptr and Issigned. Still some code to
clean up for Isint, Isfloat, and Iscomplex.

CL produced mechanically using gofmt -w -r.

Passes toolstash -cmp.

Change-Id: If4f807bb7f2b357288d2547be2380eb511875786
Reviewed-on: https://go-review.googlesource.com/21339
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
diff --git a/src/cmd/compile/internal/gc/bexport.go b/src/cmd/compile/internal/gc/bexport.go
index 24812ab..22de0a6 100644
--- a/src/cmd/compile/internal/gc/bexport.go
+++ b/src/cmd/compile/internal/gc/bexport.go
@@ -635,7 +635,7 @@
 
 func basetypeName(t *Type) string {
 	s := t.Sym
-	if s == nil && Isptr[t.Etype] {
+	if s == nil && t.IsPtr() {
 		s = t.Elem().Sym // deref
 	}
 	if s != nil {