[ARM] Add missing patterns for DSP muls

[ARM] Add missing patterns for DSP muls

Using a PatLeaf for sext_16_node allowed matching smulbb and smlabb
instructions once the operands had been sign extended. But we also
need to use sext_inreg operands along with sext_16_node to catch a
few more cases that enable use to remove the unnecessary sxth.

Differential Revision: https://reviews.llvm.org/D55992

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350613 91177308-0d34-0410-b5e6-96231b3b80d8