[RISCV] Allow fractional LMUL for reduction start value

For reductions, we need to put the start value into a source vector. For fractional LMULs, we can perform the operation at the original LMUL.  For LMUL > 1, we eventually want to use a scalar insert, but that's outside the scope of this patch.

Differential Revision: https://reviews.llvm.org/D139747