[llvm] [DAG] getNode - fold (sext (trunc x)) -> x iff the upper bits are already signbits (PR #151945)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

The fold often fails if the trunc gets folded further up the dag chain before the sext combines are called - it gets even worse in some of the topological sorting work I'm currently investigating.

https://github.com/llvm/llvm-project/pull/151945