[llvm] [SelectionDAG] Remove unneeded assert from SelectionDAG::getSignedConstant. NFC (PR #114336)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM, though the `implicitTrunc=false` is no longer needed now that https://github.com/llvm/llvm-project/pull/114539 has landed again (and it looks like it is sticking this time).

https://github.com/llvm/llvm-project/pull/114336