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

runtime/vm/simulator_arm64.cc

Index: runtime/vm/simulator_arm64.cc
===================================================================
--- runtime/vm/simulator_arm64.cc	(revision 42607)
+++ runtime/vm/simulator_arm64.cc	(working copy)
@@ -2530,6 +2530,15 @@
         static_cast<__int128>(rn_val) * static_cast<__int128>(rm_val);
     const int64_t alu_out = static_cast<int64_t>(res >> 64);
     set_register(instr, rd, alu_out, R31IsZR);
+  } else if ((instr->Bits(29, 2) == 0) && (instr->Bits(21, 3) == 6) &&
+             (instr->Bit(15) == 0)) {
+    // Format(instr, "umulh 'rd, 'rn, 'rm");
+    const uint64_t rn_val = get_register(rn, R31IsZR);
+    const uint64_t rm_val = get_register(rm, R31IsZR);
+    const __int128 res =
+        static_cast<__int128>(rn_val) * static_cast<__int128>(rm_val);
+    const int64_t alu_out = static_cast<int64_t>(res >> 64);
+    set_register(instr, rd, alu_out, R31IsZR);
   } else if ((instr->Bits(29, 3) == 4) && (instr->Bits(21, 3) == 5) &&
              (instr->Bit(15) == 0)) {
     // Format(instr, "umaddl 'rd, 'rn, 'rm, 'ra");