Add unsigned type iv_cand for iv_use with non mode-precision type

Add unsigned type iv_cand for iv_use with non mode-precision type

Precisely,  for iv_use if it's not integer/pointer type, or non-mode
precision type, add candidate for the corresponding scev in unsigned
type with the same precision, rather than its original type.

Backport from master.
2020-04-09  Bin Cheng  <bin.cheng@linux.alibaba.com>
    PR tree-optimization/93674

gcc/
    * tree-ssa-loop-ivopts.c (langhooks.h): New include.
    (add_iv_candidate_for_use): For iv_use of non integer or pointer type,
    or non-mode precision type, add candidate in unsigned type with the
    same precision.

gcc/testsuite/
    * g++.dg/pr93674.C: New test.