[InstCombine] add tests for udiv with common factor; NFC

This fold is mentioned in PR38239:
https://bugs.llvm.org/show_bug.cgi?id=38239

The general case probably belongs in -reassociate, but given that we do 
basic reassociation optimizations similar to this in instcombine already, 
we might as well be consistent within instcombine and handle this pattern?

llvm-svn: 338038