[SimplifyIndVar] Canonicalize comparisons to unsigned while eliminating truncs

This is a follow-up for the patch rL335020. When we replace compares against
trunc with compares against wide IV, we can also replace signed predicates with
unsigned where it is legal.

Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D48763

llvm-svn: 338115