[ConstraintElim] Move after first instcombine run.

Running ConstraintEliminiation after the first InstCombine run results
in slightly more simplifications on average.

There are is a tiny number of regressions, mostly due to CVP eliminating
a condition that ConstraintElimination would use, but in most cases
there's a slight improvement or no change.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D140853