Assert that only reg- or word-sized values are byte swapped

Targets and hosts today are little-endian (arm, x86), so this change
should be a no-op as they will not encounter the byte swapping cases.

Byte swapping  will happen when cross debugging of big endian-targets
(e.g. MIPS, PPC) on a little-endian host (x86).  Register- or word-
sized data copies need to be swapped, but calls to ExtractBytes or
CopyByteOrderedData that would invoke the swapping case are presumably
in error.

llvm-svn: 191005