[llvm] [RISCV] Explicitly check for supported opcodes in optimizeCondBranch. NFC (PR #145622)

[Craig Topper via llvm-commits]

topperc wrote:

> > Ultimately I want to try to fix that the RISCVCC enum has turned into a proxy for opcode by having an entry for each vendor's branch instructions.
> 
> Do you have a view on where you're trying to go? Just having the conditions, or also having e.g. both `equal` and `equal to immediate`?

I'm considering making analyzeBranch return the raw opcode instead of the condition code enum. And make the reverseBranchCondition work on raw opcodes.

I would reduce the condition code enum back down to 6 values.

This would leave select instructions as the primary user of the condition code enum. My thought is we could use the select opcode + condition code enum to expand the select into branches.

It might still be useful to translate an opcode into the 6 condition codes like we currently need to do for evaluateCondBranch. So I may leave getCondFromBranchOpc but reduce it to only return the 6 possible values.

https://github.com/llvm/llvm-project/pull/145622