cmd/compile/internal/ssa: inline memmove with known size

Replace calls to memmove with known (constant) size, with OpMove.
Do it only if it is safe from aliasing point of view.
Helps with code like this:

append(buf,"const str"...)

In strconv this provides nice benefit:
Quote-6                                   731ns ± 2%   647ns ± 3%  -11.41%  (p=0.000 n=10+10)
QuoteRune-6                               117ns ± 5%   111ns ± 1%   -4.54%  (p=0.000 n=10+10)
AppendQuote-6                             475ns ± 0%   396ns ± 0%  -16.59%  (p=0.000 n=9+10)
AppendQuoteRune-6                        32.0ns ± 0%  27.4ns ± 0%  -14.41%  (p=0.000 n=8+9)

Change-Id: I7704f5c51b46aed2d8f033de74c75140fc35036c
Reviewed-on: https://go-review.googlesource.com/54394
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/ssa/rewrite_test.go

@@ -103,3 +103,25 @@ func TestLog2(t *testing.T) {
 		}
 	}
 }
+
+// We generate memmove for copy(x[1:], x[:]), however we may change it to OpMove,
+// because size is known. Check that OpMove is alias-safe, or we did call memmove.
+func TestMove(t *testing.T) {
+	x := [...]byte{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40}
+	copy(x[1:], x[:])
+	for i := 1; i < len(x); i++ {
+		if int(x[i]) != i {
+			t.Errorf("Memmove got converted to OpMove in alias-unsafe way. Got %d insted of %d in position %d", int(x[i]), i, i+1)
+		}
+	}
+}
+
+func TestMoveSmall(t *testing.T) {
+	x := [...]byte{1, 2, 3, 4, 5, 6, 7}
+	copy(x[1:], x[:])
+	for i := 1; i < len(x); i++ {
+		if int(x[i]) != i {
+			t.Errorf("Memmove got converted to OpMove in alias-unsafe way. Got %d instead of %d in position %d", int(x[i]), i, i+1)
+		}
+	}
+}