cmd/compile: base mapaccess optimizations on algtype

algtype already controls the behavior of the normal map access code
paths, so it makes sense to base the decision on which optimized paths
are applicable on it too.

Enables use of optimized paths for key types like [8]byte and struct{s
string}.

Fixes #13271.

Change-Id: I48c52d97abaa7259ad5aba9641ea996a967cd359
Reviewed-on: https://go-review.googlesource.com/17464
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -787,14 +787,12 @@ opswitch:
 		t := r.Left.Type
 		p := ""
 		if t.Type.Width <= 128 { // Check ../../runtime/hashmap.go:maxValueSize before changing.
-			switch Simsimtype(t.Down) {
-			case TINT32, TUINT32:
+			switch algtype(t.Down) {
+			case AMEM32:
 				p = "mapaccess2_fast32"
-
-			case TINT64, TUINT64:
+			case AMEM64:
 				p = "mapaccess2_fast64"
-
-			case TSTRING:
+			case ASTRING:
 				p = "mapaccess2_faststr"
 			}
 		}
@@ -1203,14 +1201,12 @@ opswitch:
 		t := n.Left.Type
 		p := ""
 		if t.Type.Width <= 128 { // Check ../../runtime/hashmap.go:maxValueSize before changing.
-			switch Simsimtype(t.Down) {
-			case TINT32, TUINT32:
+			switch algtype(t.Down) {
+			case AMEM32:
 				p = "mapaccess1_fast32"
-
-			case TINT64, TUINT64:
+			case AMEM64:
 				p = "mapaccess1_fast64"
-
-			case TSTRING:
+			case ASTRING:
 				p = "mapaccess1_faststr"
 			}
 		}