[IR] Add PHINode::removeIncomingValueIf() (NFC)

Add an API that allows removing multiple incoming phi values based
on a predicate callback, as suggested on D157621.

This makes sure that the removal is linear time rather than quadratic,
and avoids subtleties around iterator invalidation.

I have replaced some of the more straightforward users with the new
API, though there's a couple more places that should be able to use it.

Differential Revision: https://reviews.llvm.org/D158064