[ARM] GlobalISel: Legalize s8 and s16 G_(S|U)DIV

We have to widen the operands to 32 bits and then we can either use
hardware division if it is available or lower to a libcall otherwise.

At the moment it is not enough to set the Legalizer action to
WidenScalar, since for libcalls it won't know what to do (it won't be
able to find what size to widen to, because it will find Libcall and not
Legal for 32 bits). To hack around this limitation, we request Custom
lowering, and as part of that we widen first and then we run another
legalizeInstrStep on the widened DIV.

llvm-svn: 301166