[CodeGen] Let computeKnownBits do something sensible for scalable vectors

[CodeGen] Let computeKnownBits do something sensible for scalable vectors

Until we have a real need for computing known bits for scalable
vectors I have simply changed the code to bail out for now and
pretend we know nothing. I've also fixed up some simple callers of
computeKnownBits too.

Differential Revision: https://reviews.llvm.org/D80437