test/fixedbugs: add more test cases to issue #27718

This CL add test cases for the unary FP negative
operation.

Change-Id: I54e7292ca9df05da0c2b113adefc97ee1e94c6e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/190937
Run-TryBot: Ben Shi <powerman1st@163.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

test/fixedbugs/issue27718.go

@@ -36,6 +36,20 @@ func testSub64() {
 	}
 }
 
+//go:noinline
+func neg64(x float64) float64 {
+	return -x
+}
+
+func testNeg64() {
+	var zero float64
+	inf := 1.0 / zero
+	negZero := -1 / inf
+	if 1/neg64(negZero) != inf {
+		panic("-negZero != posZero (64 bit)")
+	}
+}
+
 //go:noinline
 func add32(x float32) float32 {
 	return x + 0
@@ -64,9 +78,25 @@ func testSub32() {
 	}
 }
 
+//go:noinline
+func neg32(x float32) float32 {
+	return -x
+}
+
+func testNeg32() {
+	var zero float32
+	inf := 1.0 / zero
+	negZero := -1 / inf
+	if 1/neg32(negZero) != inf {
+		panic("-negZero != posZero (32 bit)")
+	}
+}
+
 func main() {
 	testAdd64()
 	testSub64()
+	testNeg64()
 	testAdd32()
 	testSub32()
+	testNeg32()
 }