[RISCV] Ensure small mask BUILD_VECTORs aren't expanded

The default expansion for BUILD_VECTORs -- save for going through
shuffles -- is to go through the stack. This method only works when the
type is at least byte-sized, so for v2i1 and v4i1 we would crash.

This patch ensures that small mask-type BUILD_VECTORs are always handled
without crashing. We lower to a SETCC of the equivalent i8 type.

This also exposes some pre-existing issues where the lowering when
optimizing for size results in larger code than without. Those will be
tackled in future patches.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D102767