[llvm] [DAG] fix wrong type check in DAGCombiner::visitSRA (PR #153762)

[via llvm-commits]

woruyu wrote:

I tried, but finding a case where `SIGN_EXTEND` to `TruncVT` is `LegalOrCustom` and `TRUNCATE` from VT is also `LegalOrCustom` (and still reaches instruction selection to trigger the error) feels like searching for a needle in a haystack. For a lit test, do you have any suggestions, @akiva-pripas, or would it make more sense to add a unit test instead, and if so, in which file?

https://github.com/llvm/llvm-project/pull/153762