[RISCV] Add support for fixed vector masked gather/scatter.

I've split the gather/scatter custom handler to avoid complicating
it with even more differences between gather/scatter.

Tests are the scalable vector tests with the vscale removed and
dropped the tests that used vector.insert. We're probably not
as thorough on the splitting cases since we use 128 for VLEN here
but scalable vector use a known min size of 64.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D98991