cmd/compile: add fma intrinsic for amd64

To permit ssa-level optimization, this change introduces an amd64 intrinsic
that generates the VFMADD231SD instruction for the fused-multiply-add
operation on systems that support it. System support is detected via
cpu.X86.HasFMA. A rewrite rule can then translate the generic ssa intrinsic
("Fma") to VFMADD231SD.

The benchmark compares the software implementation (old) with the intrinsic
(new).

name   old time/op  new time/op  delta
Fma-4  27.2ns ± 1%   1.0ns ± 9%  -96.48%  (p=0.008 n=5+5)

Updates #25819.

Change-Id: I966655e5f96817a5d06dff5942418a3915b09584
Reviewed-on: https://go-review.googlesource.com/c/go/+/137156
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -743,6 +743,7 @@ const (
 	OpAMD64POPCNTL
 	OpAMD64SQRTSD
 	OpAMD64ROUNDSD
+	OpAMD64VFMADD231SD
 	OpAMD64SBBQcarrymask
 	OpAMD64SBBLcarrymask
 	OpAMD64SETEQ
@@ -9625,6 +9626,22 @@ var opcodeTable = [...]opInfo{
 			},
 		},
 	},
+	{
+		name:         "VFMADD231SD",
+		argLen:       3,
+		resultInArg0: true,
+		asm:          x86.AVFMADD231SD,
+		reg: regInfo{
+			inputs: []inputInfo{
+				{0, 4294901760}, // X0 X1 X2 X3 X4 X5 X6 X7 X8 X9 X10 X11 X12 X13 X14 X15
+				{1, 4294901760}, // X0 X1 X2 X3 X4 X5 X6 X7 X8 X9 X10 X11 X12 X13 X14 X15
+				{2, 4294901760}, // X0 X1 X2 X3 X4 X5 X6 X7 X8 X9 X10 X11 X12 X13 X14 X15
+			},
+			outputs: []outputInfo{
+				{0, 4294901760}, // X0 X1 X2 X3 X4 X5 X6 X7 X8 X9 X10 X11 X12 X13 X14 X15
+			},
+		},
+	},
 	{
 		name:   "SBBQcarrymask",
 		argLen: 1,