[ARM] Improve smul* and smla* isel for Thumb2

Added (sra (shl x, 16), 16) to the sext_16_node PatLeaf for ARM to
simplify some pattern matching. This has allowed several patterns
for smul* and smla* to be removed as well as making it easier to add
the matching for the corresponding instructions for Thumb2 targets.
Also added two Pat classes that are predicated on Thumb2 with the
hasDSP flag and UseMulOps flags. Updated the smul codegen test with
the wider range of patterns plus the ThumbV6 and ThumbV6T2 targets.

Differential Revision: https://reviews.llvm.org/D22908

llvm-svn: 277450