[PPC64] Helpers for read/write an instr while relocating a half16 type. [NFC]

There are a growing number of places when we either want to read or write an
instruction when handling a half16 relocation type. On big-endian the buffer
pointer is pointing into the middle of the word we want and on little-endian it
is pointing to the start of the word. These 2 helpers are to simplify reading
and writing in these contexts.

Differential Revision: https://reviews.llvm.org/D52115

llvm-svn: 342410