cmd/compile: remove unnecessary nil-check

Removes unnecessary nil-check when referencing offset from an
address. Suggested by Keith Randall in golang#27180.

Updates golang#27180

Change-Id: I326ed7fda7cfa98b7e4354c811900707fee26021
Reviewed-on: https://go-review.googlesource.com/131735
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/ssa/nilcheck.go

@@ -47,7 +47,7 @@ func nilcheckelim(f *Func) {
 			// a value resulting from taking the address of a
 			// value, or a value constructed from an offset of a
 			// non-nil ptr (OpAddPtr) implies it is non-nil
-			if v.Op == OpAddr || v.Op == OpLocalAddr || v.Op == OpAddPtr {
+			if v.Op == OpAddr || v.Op == OpLocalAddr || v.Op == OpAddPtr || v.Op == OpOffPtr {
 				nonNilValues[v.ID] = true
 			}
 		}

test/nilptr3.go

@@ -246,8 +246,8 @@ type TT struct {
 
 func f(t *TT) *byte {
 	// See issue 17242.
-	s := &t.SS  // ERROR "removed nil check"
-	return &s.x // ERROR "generated nil check"
+	s := &t.SS  // ERROR "generated nil check"
+	return &s.x // ERROR "removed nil check"
 }
 
 // make sure not to do nil check for newobject