[ARM] Enable SMLAW[B|T] and SMLUW[B|T] instruction selection

Added ISelDAGToDAG functions to enable selection of the smlawb, smlawt,
smulwb and smulwt instructions for the ARM backend. Also updated the smul
CodeGen test and removed the smulw one.

Differential Revision: http://reviews.llvm.org/D18892

llvm-svn: 265793