avutil/mips: Restructure of generic macros

This patch includes restructuring of existing macros and addition of more generic macros.

This change was necessary to avoid repeated review comments in remaining patches which we were about to submit.
Also this patch reduces number of code lines due to maximum use of generic macros, allows better code alignment & readability etc.

These modifications in commonly used .libavutil/mips/generic_macros_msa.h. impacts the already accepted code, hence re-submitting it in 2/4,3/4 & 4/4.
Overall, this patch set is just upgrading the code with styling changes and will bring it in sync with MIPS-SIMD optimized latest codebase at our end.
Signed-off-by: Shivraj Patil <shivraj.patil@imgtec.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>