test/codegen: add Mul2 and DivPow2 test for loong64

Change-Id: I29ccd105c5418955146a3f4873162963da489a70
Reviewed-on: https://go-review.googlesource.com/c/go/+/697935
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

Author: sophie-zhao

test/codegen/floats.go

@@ -19,6 +19,7 @@ func Mul2(f float64) float64 {
 	// amd64:"ADDSD",-"MULSD"
 	// arm/7:"ADDD",-"MULD"
 	// arm64:"FADDD",-"FMULD"
+	// loong64:"ADDD",-"MULD"
 	// ppc64x:"FADD",-"FMUL"
 	// riscv64:"FADDD",-"FMULD"
 	return f * 2.0
@@ -29,6 +30,7 @@ func DivPow2(f1, f2, f3 float64) (float64, float64, float64) {
 	// amd64:"MULSD",-"DIVSD"
 	// arm/7:"MULD",-"DIVD"
 	// arm64:"FMULD",-"FDIVD"
+	// loong64:"MULD",-"DIVD"
 	// ppc64x:"FMUL",-"FDIV"
 	// riscv64:"FMULD",-"FDIVD"
 	x := f1 / 16.0
@@ -37,6 +39,7 @@ func DivPow2(f1, f2, f3 float64) (float64, float64, float64) {
 	// amd64:"MULSD",-"DIVSD"
 	// arm/7:"MULD",-"DIVD"
 	// arm64:"FMULD",-"FDIVD"
+	// loong64:"MULD",-"DIVD"
 	// ppc64x:"FMUL",-"FDIVD"
 	// riscv64:"FMULD",-"FDIVD"
 	y := f2 / 0.125
@@ -45,6 +48,7 @@ func DivPow2(f1, f2, f3 float64) (float64, float64, float64) {
 	// amd64:"ADDSD",-"DIVSD",-"MULSD"
 	// arm/7:"ADDD",-"MULD",-"DIVD"
 	// arm64:"FADDD",-"FMULD",-"FDIVD"
+	// loong64:"ADDD",-"MULD",-"DIVD"
 	// ppc64x:"FADD",-"FMUL",-"FDIV"
 	// riscv64:"FADDD",-"FMULD",-"FDIVD"
 	z := f3 / 0.5