[Inliner] Handle convergence control when inlining a call

When a convergencectrl token is passed to a convergent call, and the called
function in turn calls the entry intrinsic, the intrinsic is now now replaced
with the convergencectrl token.

The spec requires the following check:
  A call from function F to function G can be inlined only if:
  - at least one of F or G does not make any convergent calls, or,
  - both F and G make the same kind of convergent calls: controlled or
    uncontrolled.

But this change does not implement this complete check. A proper implemenation
require a whole new analysis that identifies convergence in every function. For
now, we skip that and just do a cursory check for the entry intrinsic. The
underlying assumption is that in a compiler flow that fully implements
convergence control tokens, there is no mixing of controlled and uncontrolled
convergent operations in the whole program.

This is a reboot of the original change D85606 by
Nicolai Haehnle <nicolai.haehnle@amd.com>.

Reviewed By: arsenm, nhaehnle

Differential Revision: https://reviews.llvm.org/D152431