cmd/compile,internal/bytealg: add MemEq intrinsic for runtime.memequal

Introduce a new MemEq SSA operation for runtime.memequal. The operation
is initially implemented for arm64. The change adds opt rules (following
existing rules for call to runtime.memequal), working with MemEq, and a
later op version LoweredMemEq which may be lowered differently for more
constant size cases in future (for other targets as well as for arm64).
The new MemEq SSA operation does not have memory result, allowing cse of
loads operations around it.

Code size difference (for arm64 linux):

Executable            Old .text  New .text     Change
-------------------------------------------------------
asm                     1970420    1969668     -0.04%
cgo                     1741220    1740212     -0.06%
compile                 8956756    8959428     +0.03%
cover                   1879332    1878772     -0.03%
link                    2574116    2572660     -0.06%
preprofile               867124     866820     -0.04%
vet                     2890404    2888596     -0.06%

Change-Id: I6ab507929b861884d17d5818cfbd152cf7879751
Reviewed-on: https://go-review.googlesource.com/c/go/+/686655
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

test/codegen/comparisons.go

@@ -660,6 +660,17 @@ func equalVarString8(a string) bool {
 	return a[:8] == b
 }
 
+func equalVarStringNoSpill(a,b string) bool {
+	s := string("ZZZZZZZZZ")
+	// arm64:".*memequal"
+	memeq1 := a[:9] == s
+	// arm64:-".*"
+	memeq2 := s == a[:9]
+	// arm64:-"MOVB\tR0,.*SP",".*memequal"
+	memeq3 := s == b[:9]
+	return memeq1 && memeq2 && memeq3
+}
+
 func cmpToCmn(a, b, c, d int) int {
 	var c1, c2, c3, c4, c5, c6, c7, c8, c9, c10, c11 int
 	// arm64:`CMN`,-`CMP`