[SimplifyLibCalls] Skip unused calls in sincos transform

If the call result is unused, we should let it get DCEd rather
than replacing it. Also, don't try to replace an existing sincos
with another one (unless it's as part of combining sin and cos).

This avoids an infinite combine loop if the calls are not DCEd
as expected, which can happen with D94106 and lack of willreturn
annotation in hand-crafted IR.