[InstCombine] add folds for not-of-shift-right

This is another step towards getting rid of dyn_castNotVal, 
so we can recommit:
https://reviews.llvm.org/rL300977

As the tests show, we were missing the lshr case for constants
and both ashr/lshr vector splat folds. The ashr case with constant
was being performed inefficiently in 2 steps. It's also possible
there was a latent bug in that case because we can't do that fold
if the constant is positive:
http://rise4fun.com/Alive/Bge

llvm-svn: 302465