[turbofan] fix LowerStringFromSingleCodePoint on big-endian

R=jarin@chromium.org, mstarzinger@chromium.org
Bug=v8:7651

Change-Id: I4007c6de83806c2aad8fc105331986bfcf709033
Reviewed-on: https://chromium-review.googlesource.com/1012460
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52651}

src/compiler/effect-control-linearizer.cc

@@ -3167,7 +3167,11 @@ Node* EffectControlLinearizer::LowerStringFromSingleCodePoint(Node* node) {
                                   __ Int32Constant(0xDC00));
 
         // codpoint = (trail << 16) | lead;
+#if V8_TARGET_BIG_ENDIAN
+        code = __ Word32Or(__ Word32Shl(lead, __ Int32Constant(16)), trail);
+#else
         code = __ Word32Or(__ Word32Shl(trail, __ Int32Constant(16)), lead);
+#endif
         break;
       }
     }