[InstCombine] Factor out unreachable inst idiom creation [NFC]

In InstCombine, we use an idiom of "store i1 true, i1 undef" to indicate we've found a path which we've proven unreachable.  We can't actually insert the unreachable instruction since that would require changing the CFG.  We leave that to simplifycfg later.

This just factors out that idiom creation so we don't duplicate the same mostly undocument idiom creation in multiple places.

llvm-svn: 358600