[ARM] Replace HasT2ExtractPack with HasDSP

Removed the HasT2ExtractPack feature and replaced its references
with HasDSP. This then allows the Thumb2 extend instructions to be
selected for ARMv8M +dsp. These instruction descriptions have also
been refactored and more target tests have been added for their isel.

Differential Revision: https://reviews.llvm.org/D29623

llvm-svn: 295452