[builtins] fix load of surrogate pair for BE platform

Swaping the order when loading surrogate pairs on big endian
platform. This fixes testcase string-iterator.js on big
endian.

BUG=
R=bmeurer@chromium.org, mstarzinger@chromium.org, jkummerow@chromium.org

Review-Url: https://codereview.chromium.org/2431223010
Cr-Commit-Position: refs/heads/master@{#40535}

src/builtins/builtins-string.cc

@@ -1380,7 +1380,12 @@ compiler::Node* LoadSurrogatePairInternal(CodeStubAssembler* assembler,
     switch (encoding) {
       case UnicodeEncoding::UTF16:
         var_result.Bind(assembler->WordOr(
+// Need to swap the order for big-endian platforms
+#if V8_TARGET_BIG_ENDIAN
+            assembler->WordShl(lead, assembler->Int32Constant(16)), trail));
+#else
             assembler->WordShl(trail, assembler->Int32Constant(16)), lead));
+#endif
         break;
 
       case UnicodeEncoding::UTF32: {

src/compiler/js-builtin-reducer.cc

@@ -1141,9 +1141,16 @@ Reduction JSBuiltinReducer::ReduceStringIteratorNext(Node* node) {
           {
             vtrue3 = graph()->NewNode(
                 simplified()->NumberBitwiseOr(),
+// Need to swap the order for big-endian platforms
+#if V8_TARGET_BIG_ENDIAN
+                graph()->NewNode(simplified()->NumberShiftLeft(), lead,
+                                 jsgraph()->Int32Constant(16)),
+                trail);
+#else
                 graph()->NewNode(simplified()->NumberShiftLeft(), trail,
                                  jsgraph()->Int32Constant(16)),
                 lead);
+#endif
           }
 
           Node* if_false3 = graph()->NewNode(common()->IfFalse(), branch3);