cmd/internal/obj/loong64: add support for FSEL instruction

Go asm syntax:
	FSEL	FCC, FK, FJ, FD

Equivalent platform assembler syntax:
	fsel	fd, fj, fk, ca

Change-Id: If75f16fca0adfc03f4952f8a5143d22da33ed425
Reviewed-on: https://go-review.googlesource.com/c/go/+/693457
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Meidan Li <limeidan@loongson.cn>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>

Author: sophie-zhao

src/cmd/asm/internal/asm/testdata/loong64enc1.s

@@ -376,6 +376,10 @@ lable2:
 	FTINTRNEVF	F0, F2		// 02e41a01
 	FTINTRNEVD	F0, F2		// 02e81a01
 
+	// FSEL instruction
+	FSEL	FCC0, F1, F2, F3	// 4304000d
+	FSEL	FCC1, F1, F2		// 4284000d
+
 	// LDX.{B,BU,H,HU,W,WU,D} instructions
 	MOVB		(R14)(R13), R12	// cc350038
 	MOVBU		(R14)(R13), R12	// cc352038

src/cmd/internal/obj/loong64/a.out.go

@@ -748,6 +748,9 @@ const (
 	AFTINTRNEVF
 	AFTINTRNEVD
 
+	// 3.2.4.2
+	AFSEL
+
 	// LSX and LASX memory access instructions
 	AVMOVQ
 	AXVMOVQ

src/cmd/internal/obj/loong64/anames.go

@@ -154,6 +154,9 @@ var optab = []Optab{
 	{AFMADDF, C_FREG, C_FREG, C_NONE, C_FREG, C_NONE, 37, 4, 0, 0},
 	{AFMADDF, C_FREG, C_FREG, C_FREG, C_FREG, C_NONE, 37, 4, 0, 0},
 
+	{AFSEL, C_FCCREG, C_FREG, C_FREG, C_FREG, C_NONE, 33, 4, 0, 0},
+	{AFSEL, C_FCCREG, C_FREG, C_NONE, C_FREG, C_NONE, 33, 4, 0, 0},
+
 	{AMOVW, C_REG, C_NONE, C_NONE, C_SAUTO, C_NONE, 7, 4, REGSP, 0},
 	{AMOVWU, C_REG, C_NONE, C_NONE, C_SAUTO, C_NONE, 7, 4, REGSP, 0},
 	{AMOVV, C_REG, C_NONE, C_NONE, C_SAUTO, C_NONE, 7, 4, REGSP, 0},
@@ -1517,6 +1520,7 @@ func buildop(ctxt *obj.Link) {
 			AWORD,
 			APRELD,
 			APRELDX,
+			AFSEL,
 			obj.ANOP,
 			obj.ATEXT,
 			obj.AFUNCDATA,
@@ -2387,6 +2391,16 @@ func (c *ctxt0) asmout(p *obj.Prog, o *Optab, out []uint32) {
 		}
 		o1 = OP_6IRR(c.opirr(p.As), uint32(v), uint32(r), uint32(p.To.Reg))
 
+	case 33: // fsel ca, fk, [fj], fd
+		ca := uint32(p.From.Reg)
+		fk := uint32(p.Reg)
+		fd := uint32(p.To.Reg)
+		fj := fd
+		if len(p.RestArgs) > 0 {
+			fj = uint32(p.GetFrom3().Reg)
+		}
+		o1 = 0x340<<18 | (ca&0x7)<<15 | (fk&0x1F)<<10 | (fj&0x1F)<<5 | (fd & 0x1F)
+
 	case 34: // mov $con,fr
 		v := c.regoff(&p.From)
 		a := AADDU