[LV] Extend trunc optimization to all IVs with constant integer steps

This patch extends the optimization of truncations whose operand is an
induction variable with a constant integer step. Previously we were only
applying this optimization to the primary induction variable. However, the cost
model assumes the optimization is applied to the truncation of all integer
induction variables (even regardless of step type). The transformation is now
applied to the other induction variables, and I've updated the cost model to
ensure it is better in sync with the transformation we actually perform.

Differential Revision: https://reviews.llvm.org/D29847

llvm-svn: 294967