cmd/gc: test temp string comparison with all ops

The comment on `slicebytetostringtmp` mention that `==` operator does
not allocate []byte to string conversion, but the test was testing only
`==` and `!=` and the compiler actually optimizes all comparison
operators.

Also added a test for concatenation comparison, which also should not
allocate.

Change-Id: I6f4c5c4f238808138fa901732e1fd5b6ab25f725
Reviewed-on: https://go-review.googlesource.com/c/go/+/456415
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: colega

src/runtime/string_test.go

@@ -223,17 +223,44 @@ func TestLargeStringConcat(t *testing.T) {
 	}
 }
 
+func TestConcatTempString(t *testing.T) {
+	s := "bytes"
+	b := []byte(s)
+	n := testing.AllocsPerRun(1000, func() {
+		if "prefix "+string(b)+" suffix" != "prefix bytes suffix" {
+			t.Fatalf("strings are not equal: '%v' and '%v'", "prefix "+string(b)+" suffix", "prefix bytes suffix")
+		}
+	})
+	if n != 0 {
+		t.Fatalf("want 0 allocs, got %v", n)
+	}
+}
+
 func TestCompareTempString(t *testing.T) {
 	s := strings.Repeat("x", sizeNoStack)
 	b := []byte(s)
 	n := testing.AllocsPerRun(1000, func() {
 		if string(b) != s {
 			t.Fatalf("strings are not equal: '%v' and '%v'", string(b), s)
 		}
+		if string(b) < s {
+			t.Fatalf("strings are not equal: '%v' and '%v'", string(b), s)
+		}
+		if string(b) > s {
+			t.Fatalf("strings are not equal: '%v' and '%v'", string(b), s)
+		}
 		if string(b) == s {
 		} else {
 			t.Fatalf("strings are not equal: '%v' and '%v'", string(b), s)
 		}
+		if string(b) <= s {
+		} else {
+			t.Fatalf("strings are not equal: '%v' and '%v'", string(b), s)
+		}
+		if string(b) >= s {
+		} else {
+			t.Fatalf("strings are not equal: '%v' and '%v'", string(b), s)
+		}
 	})
 	if n != 0 {
 		t.Fatalf("want 0 allocs, got %v", n)