Move asserts from PHINode::addIncoming to PHINode::setIncoming*

We had asserts in PHINode::addIncoming to check that the value types matched
the type of the PHI, and that the associated BB was not null. These did not
catch, however, later uses of setIncomingValue and setIncomingBlock (which are
called by addIncoming as well). Moving the asserts to PHINode::setIncoming*
provides better coverage. NFC.

llvm-svn: 247492