[IndVarSimplify][SimplifyIndVar] Move WidenIV to Utils/SimplifyIndVar. NFCI.

This moves WidenIV from IndVarSimplify to Utils/SimplifyIndVar so that we have
createWideIV available as a generic helper utility. I.e., this is not only
useful in IndVarSimplify, but could be useful for loop transformations. For
example, motivation for this refactoring is the loop flatten transformation: if
induction variables in a loop nest can be widened, we can avoid having to
perform certain overflow checks, enabling this transformation.

Differential Revision: https://reviews.llvm.org/D90421