cmd/compile: simplify IsNonNil ConstNil

Change-Id: I9ed5a2065cef06708e319b16c801da2eff42004e
Reviewed-on: https://go-review.googlesource.com/35497
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -650,6 +650,7 @@
 (NeqPtr (ConstNil) p) -> (IsNonNil p)
 (EqPtr p (ConstNil)) -> (Not (IsNonNil p))
 (EqPtr (ConstNil) p) -> (Not (IsNonNil p))
+(IsNonNil (ConstNil)) -> (ConstBool [0])
 
 // slice and interface comparisons
 // The frontend ensures that we can only compare against nil,

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

@@ -114,6 +114,8 @@ func rewriteValuegeneric(v *Value, config *Config) bool {
 		return rewriteValuegeneric_OpIMake(v, config)
 	case OpIsInBounds:
 		return rewriteValuegeneric_OpIsInBounds(v, config)
+	case OpIsNonNil:
+		return rewriteValuegeneric_OpIsNonNil(v, config)
 	case OpIsSliceInBounds:
 		return rewriteValuegeneric_OpIsSliceInBounds(v, config)
 	case OpLeq16:
@@ -3407,6 +3409,23 @@ func rewriteValuegeneric_OpIsInBounds(v *Value, config *Config) bool {
 	}
 	return false
 }
+func rewriteValuegeneric_OpIsNonNil(v *Value, config *Config) bool {
+	b := v.Block
+	_ = b
+	// match: (IsNonNil (ConstNil))
+	// cond:
+	// result: (ConstBool [0])
+	for {
+		v_0 := v.Args[0]
+		if v_0.Op != OpConstNil {
+			break
+		}
+		v.reset(OpConstBool)
+		v.AuxInt = 0
+		return true
+	}
+	return false
+}
 func rewriteValuegeneric_OpIsSliceInBounds(v *Value, config *Config) bool {
 	b := v.Block
 	_ = b