Fix undefined behavior (shift of negative value) in RuntimeDyldMachOAArch64::encodeAddend.

[samsonov]

Just noticed that a similar fix is already mailed in http://reviews.llvm.org/D6884

[lhames]

Apologies for not getting to this. Thanks for the patch Alexey, and thanks
Chandler for the review!

• Lang.

lib/ExecutionEngine/RuntimeDyld/Targets/RuntimeDyldMachOAArch64.h

case MachO::ARM64_RELOC_PAGE21: {
       auto *p = reinterpret_cast<support::aligned_ulittle32_t *>(LocalAddress);
       assert((*p & 0x9F000000) == 0x90000000 && "Expected adrp instruction.");
 
       // Check that the addend fits into 21 bits (+ 12 lower bits).
       assert((Addend & 0xFFF) == 0 && "ADRP target is not page aligned.");
       assert(isInt<33>(Addend) && "Invalid page reloc value.");
 
       // Encode the addend into the instruction.
-      uint32_t ImmLoValue = (uint32_t)(Addend << 17) & 0x60000000;
-      uint32_t ImmHiValue = (uint32_t)(Addend >> 9) & 0x00FFFFE0;
+      uint32_t ImmLoValue = ((uint64_t)Addend << 17) & 0x60000000;
+      uint32_t ImmHiValue = ((uint64_t)Addend >> 9) & 0x00FFFFE0;
       *p = (*p & 0x9F00001F) | ImmHiValue | ImmLoValue;
       break;
     }
     case MachO::ARM64_RELOC_GOT_LOAD_PAGEOFF12: {
       // Verify that the relocation points to one of the expected load / store
       // instructions.
       auto *p = reinterpret_cast<support::aligned_ulittle32_t *>(LocalAddress);
       assert((*p & 0x3B000000) == 0x39000000 &&