This removes creation of udiv/sdiv/urem/srem constant expressions,
in preparation for their removal. I've added a
ConstantExpr::isDesirableBinOp() predicate to determine whether
an expression should be created for a certain operator.
With this patch, div/rem expressions can still be created through
explicit IR/bitcode, forbidding them entirely will be the next step.
Differential Revision: https://reviews.llvm.org/D128820
935570b2ad