[ARM] Add v8m.base pattern for add negative imm

The v8m.base ISA contains movw, which can operate on an unsigned
16-bit value. Add the pattern that converts an add with a negative
value, that could fit into 16-bits when negated, into a sub with that
positive value.

Differential Revision: https://reviews.llvm.org/D57942

llvm-svn: 353692