[RISCV] Disable matchSplatAsGather for i1 vectors to prevent creating illegal nodes.

We were incorrectly creating a VRGATHER node with i1 vector type. We
could support this by promoting the mask to i8 and truncating it, but
for now I want to prevent the crash.

Fixes PR56007.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D127681