Hexagon: Paper over the undefined behaviour introduced by r238692

This stops shifting a 32-bit value by such absurd amounts as 96 and
120. We do this by dropping a call to the function that was doing this
entirely, which rather surprisingly doesn't break *any* tests.

I've also added an assert in the misbehaving function to prove that
it's no longer being called with completely invalid arguments.

This change looks pretty bogus and we should probably be reverting
r238692 instead, but this is hard to do with the number of follow ups
that have happened since. It can't be any worse than the undefined
behaviour that was happening before though.

llvm-svn: 240526