[RISCV] Change zext.w to be an alias of add.uw rd, rs1, x0 instead of pack.

This didn't make it into the published 0.93 spec, but it was the
intention.

But it is in the tex source as of this commit
https://github.com/riscv/riscv-bitmanip/commit/d172f029c074d47026a0c0d0f12d8b475c86a472

This means zext.w now requires Zba. Not sure if we should still use
pack if Zbp is enabled and Zba isn't. I'll leave that for the future
when pack is closer to being final.

Reviewed By: asb, frasercrmck

Differential Revision: https://reviews.llvm.org/D94736