[ELF][PPC] Simplify {read,write}FromHalf16

I've change the variable names used in PPC64.cpp from "Instr" to "Insn"
because "Insn" is a more common abbreviation for "instruction".

While changing PPC64.cpp relocateOne(), make R_PPC64_ADDR16_LO{_DS}
slightly more efficient by saving a read and a write for the TocOptimize
case.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D63043

llvm-svn: 362867