[llvm] [TTI] Check type legalization of both src and result for fpto{u|s}i.sat. (PR #147657)

[Elvis Wang via llvm-commits]

ElvisWang123 wrote:

> This fixes the case where one or both of the types is invalid. But don't we need to take the maximum of the two `LT.second` values when both types are valid but one them splits more than the other?

I think would be better to leave this into follow-up PR.

https://github.com/llvm/llvm-project/pull/147657