[llvm] [DAG] Remove restrictions and increase optimization opportunities (PR #68972)

[via llvm-commits]

goldsteinn wrote:

1. Can you split the new tests into a seperate commit s.t we can see the diff this patch generates.
2. "Remove restrictions" is a bit broad of a title for a patch only concerned with `(shl (sext (add_nsw x, c1)), c2)`. Maybe something along the lines of "Remove OneUse restriction on `sext` when folding `(shl (sext (add_nsw x, c1)), c2)`"? 

https://github.com/llvm/llvm-project/pull/68972