Recommit r233116 better: Remove a redundant instcombine involving bitcasts of geps of bitcasts

This just didn't need to be here at all, but the assertion I tried to
add wasn't appropriate either - the circumstance isn't impossible, it's
just not important to deal with it here - the gep-rooted version of this
instcombine will handle this case, we don't need to duplicate it for the
case where the gep happens to be used in a bitcast.

llvm-svn: 233404