target/mips: Refactor and fix COPY_S.<B|H|W|D> instructions

target/mips: Refactor and fix COPY_S.<B|H|W|D> instructions

The old version of the helper for the COPY_S.<B|H|W|D> MSA instructions
has been replaced with four helpers that don't use switch, and change
the endianness of the given index, when executed on a big endian host.

Signed-off-by: Mateja Marjanovic <mateja.marjanovic@rt-rk.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Message-Id: <1554212605-16457-4-git-send-email-mateja.marjanovic@rt-rk.com>