[llvm] [DAGCombine] Remove oneuse restrictions for RISCV in folding (shl (add_nsw x, c1)), c2) and folding (shl(sext(add x, c1)), c2) in some scenarios (PR #101294)

[via llvm-commits]

LiqinWeng wrote:

> Why do you override PowerPC and Hexagon's isDesirableToCommuteWithShift while they weren't previously overridden? Does the new behavior break their tests? If that's the case what about other targets?

I actually didn't change the logic. I just moved the constraints to an interface of isDesirableToCommuteWithShift to implement them.

https://github.com/llvm/llvm-project/pull/101294