[InstCombine] Preserve return attributes when merging `llvm.ptrmask`

If we have assosiated attributes i.e `([ret_attrs] (ptrmask (ptrmask
p0, m0), m1))` we should preserve `[ret_attrs]` when combining the two
`llvm.ptrmask`s.

Differential Revision: https://reviews.llvm.org/D156638