[ARM] Loop Strength Reduction crashes when targeting ARM or Thumb.

Scalar Evolution asserts when not all the operands of an Add Recurrence
Expression are loop invariants. Loop Strength Reduction should only
create affine Add Recurrences, so that both the start and the step of
the expression are loop invariants.

Differential Revision: https://reviews.llvm.org/D26185

llvm-svn: 286347