[InstCombine] add test corresponding to r324252 (PR36225); NFC

As PR36225 shows, we definitely don't want to enable the
canEvaluate* logic with phis. 

There's still a question of whether we should just revert 
r324014 completely because it exposes a compile-time sinkhole
(although that problem might exist independently).

llvm-svn: 324266