[RISCV] Don't use zero-stride vector load for gather if not optimized

We may form a zero-stride vector load when lowering gather to strided
load. As what D137699 has done, we use `load+splat` for this form if
there is no optimized implementation.
We restrict this to unmasked loads currently in consideration of the
complexity of hanlding all falses masks.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D137931