[InstCombine] add shift+logic tests (PR37098); NFC

It debateable whether instcombine should be in the business of
reassociation, but it is currently. 

These tests and PR37098 demonstrate a missing ability to do a 
simple reassociation that allows eliminating shifts. 

If we decide that functionality belongs somewhere else, then we 
should still have some tests to show that we've intentionally 
limited instcombine to *not* include this ability.

llvm-svn: 330086