[RISCV] Generate more W instructons

We rename `TuneNoStripWSuffix` to `TunePreferWInst`.

If all the users of an instruction just use the low 32 bits, we can
convert it to its W variant.

A quick test on Coremark (`-O3 -march=rv64gc`):

|        | W instructions | code size(.text) |
|--------|----------------|------------------|
| before | 302            | 12257            |
| after  | 343            | 12265            |
|        | +13.58%        | +0.065%          |

Reviewers: asb, dtcxzyw, preames, lukel97, michaelmaitland, topperc

Reviewed By: topperc, dtcxzyw

Pull Request: https://github.com/llvm/llvm-project/pull/87237