Process two 32-bit digits as one 64-bit digit in all bigint intrinsics on ARM64.

runtime/vm/assembler_arm64.h

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_ASSEMBLER_ARM64_H_
 #define VM_ASSEMBLER_ARM64_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_arm64.h directly; use assembler.h instead.
 #endif
@@ skipping 662 matching lines @@
   }
   void madd(Register rd, Register rn, Register rm, Register ra) {
     EmitMiscDP3Source(MADD, rd, rn, rm, ra, kDoubleWord);
   }
   void msub(Register rd, Register rn, Register rm, Register ra) {
     EmitMiscDP3Source(MSUB, rd, rn, rm, ra, kDoubleWord);
   }
   void smulh(Register rd, Register rn, Register rm) {
     EmitMiscDP3Source(SMULH, rd, rn, rm, R0, kDoubleWord);
   }
+  void umulh(Register rd, Register rn, Register rm) {
+    EmitMiscDP3Source(UMULH, rd, rn, rm, R0, kDoubleWord);
+  }
   void umaddl(Register rd, Register rn, Register rm, Register ra) {
     EmitMiscDP3Source(UMADDL, rd, rn, rm, ra, kDoubleWord);
   }
 
   // Move wide immediate.
   void movk(Register rd, const Immediate& imm, int hw_idx) {
     ASSERT(rd != CSP);
     const Register crd = ConcreteRegister(rd);
     EmitMoveWideOp(MOVK, crd, imm, hw_idx, kDoubleWord);
   }
@@ skipping 1275 matching lines @@
                                   Register value,
                                   Label* no_update);
 
   DISALLOW_ALLOCATION();
   DISALLOW_COPY_AND_ASSIGN(Assembler);
 };
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_ARM64_H_