AArch64: correct CodeGen of MOVZ/MOVK combinations.

According to the AArch64 ELF specification (4.6.8), it's the
assembler's responsibility to make sure the shift amount is correct in
relocated MOVZ/MOVK instructions.

This wasn't being obeyed by either the MCJIT CodeGen or RuntimeDyldELF
(which happened to work out well for JIT tests). This commit should
make us compliant in this area.