[RISCV] Use unordered indexed loads for MGATHER.

I don't think the semantics of the llvm masked gather intrinsic care
about the order the elements are loaded. For example, type legalization
by splitting will chain them in parallel. This is different than
scatter which we do chain in order.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D106025