[SDAG] Add SimplifyDemandedBits support for ISD::SPLAT_VECTOR

This improves some cases where a splat_vector uses a build_pair that can be
simplified, e.g:

(rotl x:i64, splat_vector (build_pair x1:i32, x2:i32))

rotl only demands the bottom 6 bits, so this patch allows it to simplify it to:

(rotl x:i64, splat_vector (build_pair x1:i32, undef:i32))

Which in turn improves some cases where a splat_vector_parts is lowered on
RV32.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D158839