[BypassSlowDivision] Handle division by constant numerators better.

Summary:
We don't do BypassSlowDivision when the denominator is a constant, but
we do do it when the numerator is a constant.

This patch makes two related changes to BypassSlowDivision when the
numerator is a constant:

 * If the numerator is too large to fit into the bypass width, don't
   bypass slow division (because we'll never run the smaller-width
   code).

 * If we bypass slow division where the numerator is a constant, don't
   OR together the numerator and denominator when determining whether
   both operands fit within the bypass width.  We need to check only the
   denominator.

Reviewers: tra

Subscribers: llvm-commits, jholewinski

Differential Revision: https://reviews.llvm.org/D26699

llvm-svn: 287062