Reverting r12937.

src/arm/code-stubs-arm.cc

Index: src/arm/code-stubs-arm.cc
diff --git a/src/arm/code-stubs-arm.cc b/src/arm/code-stubs-arm.cc
index 5558c55122a7cf781c60cea1b9fabd7c085d7f99..1e73a55871606f4ce7b2708b856ef6e2e9a3f9d8 100644
--- a/src/arm/code-stubs-arm.cc
+++ b/src/arm/code-stubs-arm.cc
@@ -6498,6 +6498,11 @@ void StringAddStub::Generate(MacroAssembler* masm) {
   __ tst(r4, Operand(kAsciiDataHintMask));
   __ tst(r5, Operand(kAsciiDataHintMask), ne);
   __ b(ne, &ascii_data);
+  __ eor(r4, r4, Operand(r5));
+  STATIC_ASSERT(kOneByteStringTag != 0 && kAsciiDataHintTag != 0);
+  __ and_(r4, r4, Operand(kOneByteStringTag | kAsciiDataHintTag));
+  __ cmp(r4, Operand(kOneByteStringTag | kAsciiDataHintTag));
+  __ b(eq, &ascii_data);
 
   // Allocate a two byte cons string.
   __ AllocateTwoByteConsString(r7, r6, r4, r5, &call_runtime);