go/analysis/passes/nilness: fix wrong internal code comment

I fixed wrong internal code comment.

Change-Id: I1c3e383583a7a95070c56adc7b034b6874cce5d2
GitHub-Last-Rev: 9ab1372
GitHub-Pull-Request: #502
Reviewed-on: https://go-review.googlesource.com/c/tools/+/595895
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
Reviewed-by: Tim King <[email protected]>

Author: sato-takehiro

go/analysis/passes/nilness/nilness.go

@@ -187,7 +187,7 @@ func runFunc(pass *analysis.Pass, fn *ssa.Function) {
 					// t successor learns y is nil.
 					newFacts = expandFacts(fact{binop.Y, isnil})
 				} else {
-					// x is nil, y is unknown:
+					// y is nil, x is unknown:
 					// t successor learns x is nil.
 					newFacts = expandFacts(fact{binop.X, isnil})
 				}