X64: Made bit-fiddling fallback for double-to-int32 conversion.

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
diff --git a/src/x64/assembler-x64.h b/src/x64/assembler-x64.h
index 55d082857c1c6a6ff15e0f3c8b0ec738aa05d0f6..0f06c3c448c4ca5d93f9ec522f9f0e68a07fa445 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -606,6 +606,14 @@ class Assembler : public Malloced {
     immediate_arithmetic_op(0x0, dst, src);
   }
 
+  void sbbl(Register dst, Register src) {
+    if (dst.low_bits() == 4) {  // Forces SIB byte if dst is base register.
+      arithmetic_op_32(0x19, src, dst);
+    } else {
+      arithmetic_op_32(0x1b, dst, src);
+    }
+  }
+
   void cmpb(Register dst, Immediate src) {
     immediate_arithmetic_op_8(0x7, dst, src);
   }