[SCEV] Require that addrec operands dominate the loop

SCEVExpander currently has special handling for the case where the
start or the step of an addrec do not dominate the loop header,
which is not used by any lit test.

Initially I thought that this is entirely dead code, because
addrec operands are required to be loop invariant. However,
SCEV currently allows creating an addrec with operands that are
loop invariant but defined *after* the loop.

This doesn't seem like a useful case to allow, and we don't
appear to be using this outside a single easy to adjust unit test.