cmd/compile: fix.gc.Type.cmp for map.notBucket cmp map.Bucket

Comparison of certain map types could fail to be antisymmetric.
This corrects that.

Change-Id: I88c6256053ce29950ced4ba4d538e241ee8591fe
Reviewed-on: https://go-review.googlesource.com/22552
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: jcd . <jcd@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/type.go

@@ -992,6 +992,8 @@ func (t *Type) cmp(x *Type) ssa.Cmp {
 				return ssa.CMPlt // bucket maps are least
 			}
 			return t.StructType().Map.cmp(x.StructType().Map)
+		} else if x.StructType().Map.MapType().Bucket == x {
+			return ssa.CMPgt // bucket maps are least
 		} // If t != t.Map.Bucket, fall through to general case
 
 		fallthrough