[ARM] GlobalISel: Load i1, i8 and i16 args from stack

Add support for loading i1, i8 and i16 arguments from the stack, with or without
the ABI extension flags.

When the ABI extension flags are present, we load a 4-byte value, otherwise we
preserve the size of the load and let the instruction selector replace it with a
LDRB/LDRH. This generates the same thing as DAGISel.

Differential Revision: https://reviews.llvm.org/D27803

llvm-svn: 293163