[InstCombine] Fold 'round up integer' pattern (when alignment is a power of two)

But don't deal with non-splats.

The test coverage is sufficiently exhaustive,
and alive is happy about the changes there.

Example with constants: https://alive2.llvm.org/ce/z/EUaJ5- / https://alive2.llvm.org/ce/z/Bkng2X
General proof: https://alive2.llvm.org/ce/z/3RjJ5A