Reapply CL "Option to truncate integers to 64 bits, part 2"

runtime/vm/regexp_assembler_ir.cc

Index: runtime/vm/regexp_assembler_ir.cc
diff --git a/runtime/vm/regexp_assembler_ir.cc b/runtime/vm/regexp_assembler_ir.cc
index 2d4832910c5fedeb622855aff6bb439dafd5b609..db30e92f9b7eb36765df662dd9f80207d5478dc2 100644
--- a/runtime/vm/regexp_assembler_ir.cc
+++ b/runtime/vm/regexp_assembler_ir.cc
@@ -360,8 +360,8 @@ ConstantInstr* IRRegExpMacroAssembler::Int64Constant(int64_t value) const {
 }
 
 ConstantInstr* IRRegExpMacroAssembler::Uint64Constant(uint64_t value) const {
-  return new (Z) ConstantInstr(
-      Integer::ZoneHandle(Z, Integer::NewFromUint64(value, Heap::kOld)));
+  ASSERT(value < static_cast<uint64_t>(kMaxInt64));
+  return Int64Constant(static_cast<int64_t>(value));
 }
 
 ConstantInstr* IRRegExpMacroAssembler::BoolConstant(bool value) const {