MIPS64: Fix exception return from regexp CheckStackGuardState().

Lack of sign extension on simulator builds gives bad value for
RETRY and EXCEPTION codes.

TEST=mjsunit/regexp-stack-overflow, regress-crbug-467047
BUG=v8:3992
LOG=n

Review URL: https://codereview.chromium.org/1040753003

Cr-Commit-Position: refs/heads/master@{#27553}

src/mips64/regexp-macro-assembler-mips64.cc

@@ -1146,9 +1146,9 @@ static T& frame_entry(Address re_frame, int frame_offset) {
 }
 
 
-int RegExpMacroAssemblerMIPS::CheckStackGuardState(Address* return_address,
-                                                   Code* re_code,
-                                                   Address re_frame) {
+int64_t RegExpMacroAssemblerMIPS::CheckStackGuardState(Address* return_address,
+                                                       Code* re_code,
+                                                       Address re_frame) {
   Isolate* isolate = frame_entry<Isolate*>(re_frame, kIsolate);
   StackLimitCheck check(isolate);
   if (check.JsHasOverflowed()) {

src/mips64/regexp-macro-assembler-mips64.h

@@ -90,9 +90,8 @@ class RegExpMacroAssemblerMIPS: public NativeRegExpMacroAssembler {
   // Called from RegExp if the stack-guard is triggered.
   // If the code object is relocated, the return address is fixed before
   // returning.
-  static int CheckStackGuardState(Address* return_address,
-                                  Code* re_code,
-                                  Address re_frame);
+  static int64_t CheckStackGuardState(Address* return_address, Code* re_code,
+                                      Address re_frame);
 
   void print_regexp_frame_constants();