Implement bitLength intrinsic on x64.

runtime/vm/disassembler_x64.cc

Index: runtime/vm/disassembler_x64.cc
===================================================================
--- runtime/vm/disassembler_x64.cc	(revision 43118)
+++ runtime/vm/disassembler_x64.cc	(working copy)
@@ -1522,17 +1522,17 @@
     // SETcc: Set byte on condition. Needs pointer to beginning of instruction.
     current = data + SetCC(data);
 
-  } else if ((opcode & 0xFE) == 0xA4 || (opcode & 0xFE) == 0xAC ||
-             opcode == 0xAB || opcode == 0xA3) {
+  } else if (((opcode & 0xFE) == 0xA4) || ((opcode & 0xFE) == 0xAC) ||
+             (opcode == 0xAB) || (opcode == 0xA3) || (opcode == 0xBD)) {
     // SHLD, SHRD (double-prec. shift), BTS (bit test and set), BT (bit test).
     AppendToBuffer("%s%c ", mnemonic, operand_size_code());
     int mod, regop, rm;
     get_modrm(*current, &mod, &regop, &rm);
     current += PrintRightOperand(current);
     AppendToBuffer(",%s", NameOfCPURegister(regop));
-    if (opcode == 0xAB || opcode == 0xA3) {
+    if ((opcode == 0xAB) || (opcode == 0xA3) || (opcode == 0xBD)) {
       // Done.
-    } else if (opcode == 0xA5 || opcode == 0xAD) {
+    } else if ((opcode == 0xA5) || (opcode == 0xAD)) {
       AppendToBuffer(",cl");
     } else {
       AppendToBuffer(",");
@@ -1586,6 +1586,8 @@
       return "movzxw";
     case 0xBE:
       return "movsxb";
+    case 0xBD:
+      return "bsr";
     case 0xBF:
       return "movsxw";
     case 0x12: