tests/tcg: target/mips: Extend functionality of MSA wrapper macros

Add macros that will allow testing cases when one of the source
registers is identical to the destination register.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1551800076-8104-8-git-send-email-aleksandar.markovic@rt-rk.com>