mips32: Add test cases for for mips32 DSP instruction set.

Add test cases for mips32 DSP and DSP revision 2 ASE.
Correctly model SHLL_S.PH on MIPS32.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13479 a5019735-40e9-0310-863c-91ae7b9d1cf9