[SCEV] Use backedge SCEV of PHI only if its input is loop invariant

For the PHI node

      %1 = phi [%A, %entry], [%X, %latch]

it is incorrect to use SCEV of backedge val %X as an exit value
of PHI unless %X is loop invariant.
This is because exit value of %1 is value of %X at one-before-last
iteration of the loop.

Reviewed By: Meinersbur
Differential Revision: https://reviews.llvm.org/D73181