[RISCV] Optimize vp.load with an all-ones mask

Similar to D110206, this patch optimizes unmasked vp.load intrinsics to
avoid the need of a vmset instruction to set the mask. It does so by
selecting a riscv_vle intrinsic rather than a riscv_vle_mask intrinsic.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D113022