[llvm] [RISCV][TTI] Avoid an infite recursion issue in getCastInstrCost (PR #110164)

[Philip Reames via llvm-commits]

preames wrote:

Note for reviewers - most of the cost changes are minor, and shouldn't actually matter.  The key test is the newly added @legalization_crash test.  As it's name implies, that exercises the infinite recursion bug.  

https://github.com/llvm/llvm-project/pull/110164