[SDAG][Backend] Extend the default `TLI.isTruncFree(SDValue, EVT)`

There are a few similar cases here that would surely be free across targets: for example, `isTruncFree(sext/zext x)` should return true (`x` already exists and thus is free), similarly for constants.

This should be simple enough for a beginner to add. Just search for `TargetLoweringInfo::isTruncFree`.

This would allow deduplication of code like [this](https://github.com/llvm/llvm-project/pull/176366) better across different opcodes.