[RISCV] Optimize floating point scalar move and splat

In D158086, we limit all floating point scalar move and splat can't fuse
vsetvli with different SEW, and this patch try to relax the constraint
as possible by introducing new SEW demand type:
SEWGreaterThanOrEqualAndLessThan64, that allow SEW fused with larger
SEW, but constraint it can't fused with SEW=64.

Reviewed By: rogfer01

Differential Revision: https://reviews.llvm.org/D158177