cmd/go: revert "remove unnecessary else conditions"

This reverts CL 144137.

Reason for revert: The justification for the original commit
was that golint said so, but golint is wrong. The code reads
more clearly the original way.

Change-Id: I960f286ed66fec67aabd953e7b69993f60b00bca
Reviewed-on: https://go-review.googlesource.com/c/149339Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/internal/semver/semver.go

@@ -312,8 +312,9 @@ func compareInt(x, y string) int {
 	}
 	if x < y {
 		return -1
-	}
+	} else {
 		return +1
+	}
 }
 
 func comparePrerelease(x, y string) int {
@@ -352,9 +353,10 @@ func comparePrerelease(x, y string) int {
 			if ix != iy {
 				if ix {
 					return -1
-				}
+				} else {
 					return +1
 				}
+			}
 			if ix {
 				if len(dx) < len(dy) {
 					return -1
@@ -365,14 +367,16 @@ func comparePrerelease(x, y string) int {
 			}
 			if dx < dy {
 				return -1
-			}
+			} else {
 				return +1
 			}
 		}
+	}
 	if x == "" {
 		return -1
-	}
+	} else {
 		return +1
+	}
 }
 
 func nextIdent(x string) (dx, rest string) {