[LV] Remove constant restriction for vector phi creation

We previously only created a vector phi node for an induction variable if its
step had a constant integer type. However, the step actually only needs to be
loop-invariant. We only handle inductions having loop-invariant steps, so this
patch should enable vector phi node creation for all integer induction
variables that will be vectorized.

Differential Revision: https://reviews.llvm.org/D29956

llvm-svn: 295456