[llvm] [BasicTTI] Use getTypeLegalizationCost to generalize vector cast cost. (PR #107303)

[Philip Reames via llvm-commits]

preames wrote:

> The main motivating case for this is non-power-of-2 SLP vectorization, adding @preames for RISCV

For context, I think I addressed this specifically for RISCV recently in ac6e1fd0c089043fe60bd0040ba3cad884f00206 (with a bugfix in 424b87b8d6f0fab0898cb5cfdf9b1bbf06ddee03).  

https://github.com/llvm/llvm-project/pull/107303