DAGCombiner support for UDIVREM/SDIVREM and UMUL_LOHI/SMUL_LOHI.

Check if one of the two results unneeded so see if a simpler operator
could bs used. Also check to see if each of the two computations could be
simplified if they were split into separate operators. Factor out the code
that calls visit() so that it can be used for this purpose.

llvm-svn: 42759