[SCCP] Use constant ranges for binary operators.

If one of the operands of a binary operator is a constant range, we can
use ConstantRange::binaryOp to approximate the result.

We still handle single element constant ranges as we did previously,
with ConstantExpr::get(), because ConstantRange::binaryOp still gives
worse results in a few cases for single element ranges.

Also note that we bail out early if any of the operands is still unknown.

Reviewers: davide, efriedma, mssimpso

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D71936