[RISCV] Use non-strided load if VL=1 for optimized zero stride loads

When optimizing vmv.s.x/vmv.v.x's of scalar loads, if VL is known to be
1 then we don't need to perform a stride of x0, and can just do a
regular load.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D147609