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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295063 91177308-0d34-0410-b5e6-96231b3b80d8