[InstCombine] allow more shuffle-binop folds with safe constants

The case with 2 variables is more complicated than the case where
we eliminate the shuffle entirely because a shuffle with an undef 
mask element creates an undef result. 

I'm not aware of any current analysis/transform that recognizes that 
undef propagating to a div/rem/shift, but we have to guard against 
the possibility.

llvm-svn: 336668