x64: Make sure that the upper half of a 64bit register contains 0 for int32 values.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index 80f61db7774f2455a79e703271c31c1f9d800fe8..b36f4f059332b93b0be6eeea2fc52f9bb0b07b2e 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -3278,6 +3278,8 @@ void MacroAssembler::TruncateHeapNumberToI(Register result_reg,
   }
 
   bind(&done);
+  // Keep our invariant that the upper 32 bits are zero.
+  movl(result_reg, result_reg);
 }
 
 
@@ -3294,6 +3296,8 @@ void MacroAssembler::TruncateDoubleToI(Register result_reg,
   addp(rsp, Immediate(kDoubleSize));
 
   bind(&done);
+  // Keep our invariant that the upper 32 bits are zero.
+  movl(result_reg, result_reg);
 }