Combining DIV+REM->DIVREM doesn't belong in LegalizeDAG; move it over into DAGCombiner.

Summary:
In addition to moving the code over, this patch amends the DIV,REM -> DIVREM
combining to run on all affected nodes at once: if the nodes are converted
to DIVREM one at a time, then the resulting DIVREM may get legalized by the
backend into something target-specific that we won't be able to recognize
and correlate with the remaining nodes.

The motivation is to "prepare terrain" for D13862: when we set DIV and REM
to be legalized to libcalls, instead of the DIVREM, we otherwise lose the
ability to combine them together. To prevent this, we need to take the
DIV,REM -> DIVREM combining out of the lowering stage.

Reviewers: RKSimon, eli.friedman, rengolin

Subscribers: john.brawn, rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D13733

llvm-svn: 250825