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

This reapplies commit r294967 with a fix for the execution time regressions
caught by the clang-cmake-aarch64-quick bot. We now extend the truncate
optimization to non-primary induction variables only if the truncate isn't
already free.

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

llvm-svn: 295063