Bug 1596718 - Part 1: Add MIsNullOrUndefined::foldsTo to omit unreachable tests. r=jandem

Fold away MIsNullOrUndefined when the input is definitely null/undefined resp.
never null/undefined.

Differential Revision: https://phabricator.services.mozilla.com/D53172

--HG--
extra : moz-landing-system : lando

js/src/jit/MIR.cpp

@@ -5688,6 +5688,24 @@ MDefinition* MTypedArrayIndexToInt32::foldsTo(TempAllocator& alloc) {
   return MConstant::New(alloc, Int32Value(ival));
 }
 
+MDefinition* MIsNullOrUndefined::foldsTo(TempAllocator& alloc) {
+  MDefinition* input = value();
+  if (input->isBox()) {
+    input = input->getOperand(0);
+  }
+
+  if (input->type() == MIRType::Null || input->type() == MIRType::Undefined) {
+    return MConstant::New(alloc, BooleanValue(true));
+  }
+
+  if (!input->mightBeType(MIRType::Null) &&
+      !input->mightBeType(MIRType::Undefined)) {
+    return MConstant::New(alloc, BooleanValue(false));
+  }
+
+  return this;
+}
+
 bool jit::ElementAccessIsDenseNative(CompilerConstraintList* constraints,
                                      MDefinition* obj, MDefinition* id) {
   if (obj->mightBeType(MIRType::String)) {

js/src/jit/MIR.h

@@ -10777,6 +10777,8 @@ class MIsNullOrUndefined : public MUnaryInstruction,
   TRIVIAL_NEW_WRAPPERS
   NAMED_OPERANDS((0, value))
 
+  MDefinition* foldsTo(TempAllocator& alloc) override;
+
   bool congruentTo(const MDefinition* ins) const override {
     return congruentIfOperandsEqual(ins);
   }