ARM: make MacroAssembler::LoadRoot emit exactly one instruction when in predictable code size scope.

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index 39e239a1cbf9ad82910ef1ee7ca659ea694ea027..515a0d0e32cf16c8dc0be5b5ba758e5d5a827268 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -148,13 +148,18 @@ bool LCodeGen::GeneratePrologue() {
 
 
   info()->set_prologue_offset(masm_->pc_offset());
-  // The following three instructions must remain together and unmodified for
-  // code aging to work properly.
-  __ stm(db_w, sp, r1.bit() | cp.bit() | fp.bit() | lr.bit());
-  // Add unused load of ip to ensure prologue sequence is identical for
-  // full-codegen and lithium-codegen.
-  __ LoadRoot(ip, Heap::kUndefinedValueRootIndex);
-  __ add(fp, sp, Operand(2 * kPointerSize));  // Adjust FP to point to saved FP.
+  {
+    PredictableCodeSizeScope predictible_code_size_scope(
+        masm_, kNoCodeAgeSequenceLength * Assembler::kInstrSize);
+    // The following three instructions must remain together and unmodified
+    // for code aging to work properly.
+    __ stm(db_w, sp, r1.bit() | cp.bit() | fp.bit() | lr.bit());
+    // Load undefined value here, so the value is ready for the loop
+    // below.
+    __ LoadRoot(ip, Heap::kUndefinedValueRootIndex);
+    // Adjust FP to point to saved FP.
+    __ add(fp, sp, Operand(2 * kPointerSize));
+  }
 
   // Reserve space for the stack slots needed by the code.
   int slots = GetStackSlotCount();