Revert "Revert r362867: [ELF][PPC] Simplify {read,write}FromHalf16"

This reverts commit r363060 and restores r362867.

r362867 is innocent. The ppc64le-lld-multistage-test bot failure was due to a clang/gcc .toc bug:

    ld.lld: error: relocation refers to a discarded section: .rodata._ZNK4llvm3MVT13getSizeInBitsEv
    >>> defined in utils/TableGen/CMakeFiles/llvm-tblgen.dir/CodeGenRegisters.cpp.o
    >>> referenced by CodeGenRegisters.cpp
    >>>               utils/TableGen/CMakeFiles/llvm-tblgen.dir/CodeGenRegisters.cpp.o:(.toc+0x0)

It will be worked around by D63182.

llvm-svn: 363124