[ARM] GlobalISel: Support i8/i16 ABI extensions

At the moment, this means supporting the signext/zeroext attribute on the return
type of the function. For function arguments, signext/zeroext should be handled
by the caller, so there's nothing for us to do until we start lowering calls.

Note that this does not include support for other extensions (i8 to i16), those
will be added later.

Differential Revision: https://reviews.llvm.org/D27705

llvm-svn: 293034