[SelectionDAG] Implicitly truncate known bits in SPLAT_VECTOR

Now that D139525 fixes the Hexagon infinite loop, the stopgap can be
removed to provide more information about known bits in SPLAT_VECTOR
whose operands are smaller than the bit width (which is most of the
time)

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D141075