math: remove redundant infinity tests

These cases are covered by existing comparisons against constants.

Change-Id: I19ad530e95d2437a8617f5229495da591ceb779a
Reviewed-on: https://go-review.googlesource.com/c/go/+/692255
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Sean Liao <[email protected]>
Reviewed-by: Russ Cox <[email protected]>
Auto-Submit: Sean Liao <[email protected]>

Author: Michael Munday

src/math/exp.go

@@ -109,13 +109,11 @@ func exp(x float64) float64 {
 
 	// special cases
 	switch {
-	case IsNaN(x) || IsInf(x, 1):
+	case IsNaN(x):
 		return x
-	case IsInf(x, -1):
-		return 0
-	case x > Overflow:
+	case x > Overflow: // handles case where x is +∞
 		return Inf(1)
-	case x < Underflow:
+	case x < Underflow: // handles case where x is -∞
 		return 0
 	case -NearZero < x && x < NearZero:
 		return 1 + x
@@ -157,13 +155,11 @@ func exp2(x float64) float64 {
 
 	// special cases
 	switch {
-	case IsNaN(x) || IsInf(x, 1):
+	case IsNaN(x):
 		return x
-	case IsInf(x, -1):
-		return 0
-	case x > Overflow:
+	case x > Overflow: // handles case where x is +∞
 		return Inf(1)
-	case x < Underflow:
+	case x < Underflow: // handles case where x is -∞
 		return 0
 	}