[InstCombine] don't use DeMorgan's Law on integer constants

This is the fold that causes the infinite loop in BoringSSL 
(https://github.com/google/boringssl/blob/master/crypto/cipher/e_rc2.c) 
when we fix instcombine demanded bits to prefer 'not' ops as in D32255.

There are 2 or 3 problems with dyn_castNotVal, and I don't think we can 
reinstate D32255 until dyn_castNotVal is completely eliminated.
1. As shown here, it transforms 'not' into random xor. This transform is 
   harmful to SCEV and codegen because 'not' can often be folded while 
   random xor cannot.
2. It does not transform vector constants. This is actually a good thing, 
   but if you don't believe the above argument, then we shouldn't have 
   excluded vectors.
3. It tries to avoid transforming not(not(X)). That's nice, but it doesn't
   match the greedy nature of instcombine. If we DeMorganize a pattern 
   that has an extra 'not' in it:
   ~(~(~X) & Y) --> (~X | ~Y)

   That's just another case of DeMorgan, so we should trust that we'll fold
   that pattern too:
   (~X | ~ Y) --> ~(X & Y)

Differential Revision: https://reviews.llvm.org/D32665

llvm-svn: 301923