[IndVarSimplify] Lift limitations on IV being a Phi for turn-to-invariant

These limitations are too strict, and their only purpose is to avoid code
size explosion. These restrictions seem obsolete, and the size problem
is solved in other places through cheap expansion limits.

The motivation is that the old code cannot deal with comparisons against
induction variant's increment.

Differential Revision: https://reviews.llvm.org/D138412
Reviewed By: lebedev.ri, reames