Initial steps towards cleaning up integer arithmetic IR.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index d82136f54b05f7106c689f0ae51124369ace103a..edcdc3e765d4e3224281075b3779f772e75ee456 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -16746,8 +16746,7 @@ bool Bigint::FitsIntoInt64() const {
 }
 
 
-int64_t Bigint::AsInt64Value() const {
-  ASSERT(FitsIntoInt64());
+int64_t Bigint::AsTruncatedInt64Value() const {
   const intptr_t used = Used();
   if (used == 0) return 0;
   const int64_t digit1 = (used > 1) ? DigitAt(1) : 0;
@@ -16756,6 +16755,12 @@ int64_t Bigint::AsInt64Value() const {
 }
 
 
+int64_t Bigint::AsInt64Value() const {
+  ASSERT(FitsIntoInt64());
+  return AsTruncatedInt64Value();
+}
+
+
 bool Bigint::FitsIntoUint64() const {
   ASSERT(Bigint::kBitsPerDigit == 32);
   return !Neg() && (Used() <= 2);