[ARM][GlobalISel] Legalize narrow scalar ops by widening

This is the same as r292827 for AArch64: we widen 8- and 16-bit ADD, SUB
and MUL to 32 bits since we only have TableGen patterns for 32 bits.
See the commit message for r292827 for more details.

At this point we could just remove some of the tests for regbankselect
and instruction-select, since we're not going to see any narrow
operations at those levels anymore. Instead I decided to update them
with G_ANYEXT/G_TRUNC operations, so we can validate the full sequences
generated by the legalizer.

llvm-svn: 302782