Reapply: Inline fast cases in string keyed load IC.

src/globals.h

Index: src/globals.h
diff --git a/src/globals.h b/src/globals.h
index 410cb3f8c3abab2643199c343eea695638588bf0..e3ef9589cdb23f22139e73eb6b7b126f3ed5f684 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -147,6 +147,9 @@ const int kPointerSizeLog2 = 2;
 const intptr_t kIntptrSignBit = 0x80000000;
 #endif
 
+// Mask for the sign bit in a smi.
+const intptr_t kSmiSignMask = kIntptrSignBit;
+
 const int kObjectAlignmentBits = kPointerSizeLog2;
 const intptr_t kObjectAlignment = 1 << kObjectAlignmentBits;
 const intptr_t kObjectAlignmentMask = kObjectAlignment - 1;