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


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@338038 91177308-0d34-0410-b5e6-96231b3b80d8

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