[Intrinsic] Give "is.constant" the "convergent" attribute

Summary:
Code frequently relies upon the results of "is.constant" intrinsics to
DCE invalid code paths. We don't want the intrinsic to be made control-
dependent on any additional values. For instance, we can't split a PHI
into a "constant" and "non-constant" part via jump threading in order
to "optimize" the constant part, because the "is.constant" intrinsic is
meant to return "false".

Reviewers: wmi, kazu, MaskRay

Reviewed By: kazu

Subscribers: jdoerfert, efriedma, joerg, lebedev.ri, nikic, xbolva00, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75799