[DAGCombine] Fix divrem combine not to assume div/rem type is simple.

The divrem combine assumed the type of the div/rem is simple, which isn't
necessarily true. This probably worked fine until r250825, since it only
saw legal types, but now breaks when it runs as a pre-type-legalization 
combine.

This fixes PR26835.

Differential Revision: http://reviews.llvm.org/D17878

llvm-svn: 262746