tests/tcg: target/mips: Add tests for MIPS64R6 shift instructions

Add tests for MIPS64R6 shift instructions: SLLV, SRLV, SRAV, DSLLV,
DSRLV, and DSRAV.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1551800076-8104-13-git-send-email-aleksandar.markovic@rt-rk.com>