[llvm] [RISCV] Remove -mattr=+no-rvc-hints (PR #145138)

[Alex Bradbury via llvm-commits]

https://github.com/asb approved this pull request.

I think we should remove this. I it comes partly from the presentation of RVC hint instructions being rather different than the 32-bit hints in the ISA manual: Take the example of 32-bit LUI, this is described straightforwardly as having a 5-bit `rd` and 20-bit immediate. The table of hints then explains that `lui` with rd=x0 can be used as a hint. Contrast with `c.lui` - this is defined as having an `rd` field that isn't x0 or x2. e.g. `c.lui zero, 1` isn't a `c.lui` instruction while `lui zero, 1` is an lui instruction.

There's a distinction in presentation, but I think on reflection it doesn't matter and there's no real value in the no-rvc-hints. Also, based on that initial (but I think flawed) justification, `c.ntl.*` shouldn't have been affected.

https://github.com/llvm/llvm-project/pull/145138