Revert of MIPS64: Enable shorten-64-to-32 warning.

src/mips64/lithium-codegen-mips64.cc

Index: src/mips64/lithium-codegen-mips64.cc
diff --git a/src/mips64/lithium-codegen-mips64.cc b/src/mips64/lithium-codegen-mips64.cc
index 90c4450b7779a94a548a7525c57778939db5653d..c06d80f0a5b0c646d03c309736d45848508f98b0 100644
--- a/src/mips64/lithium-codegen-mips64.cc
+++ b/src/mips64/lithium-codegen-mips64.cc
@@ -489,8 +489,8 @@
 }
 
 
-int64_t LCodeGen::ToRepresentation_donotuse(LConstantOperand* op,
-                                            const Representation& r) const {
+int32_t LCodeGen::ToRepresentation_donotuse(LConstantOperand* op,
+                                   const Representation& r) const {
   HConstant* constant = chunk_->LookupConstant(op);
   int32_t value = constant->Integer32Value();
   if (r.IsInteger32()) return value;