[llvm] [RISCV][VLOPT] Fix assertion failure across blocks (PR #124734)

[Philip Reames via llvm-commits]

preames wrote:

> It's probably worth noting that there's two types of check going on in isCandidate at the moment, those to ensure correctness (isSupportedInstr, mayRaiseFPException), and those to see if it's worthwhile (VL > 1). We could potentially move the latter into tryReduceVL to keep it alongside the other VL checks.

This seems like the right approach to me.

https://github.com/llvm/llvm-project/pull/124734