[InstCombine] remove fold that swaps xor/or with constants; NFCI

// (X ^ C1) | C2 --> (X | C2) ^ (C1&~C2)

This canonicalization was added at:
https://reviews.llvm.org/rL7264 

By moving xors out/down, we can more easily combine constants. I'm adding
tests that do not change with this patch, so we can verify that those kinds
of transforms are still happening.

This is no-functional-change-intended because there's a later fold:
// (X^C)|Y -> (X|Y)^C iff Y&C == 0
...and demanded-bits appears to guarantee that any fold that would have
hit the fold we're removing here would be caught by that 2nd fold.

Similar reasoning was used in:
https://reviews.llvm.org/rL299384

The larger motivation for removing this code is that it could interfere with 
the fix for PR32706:
https://bugs.llvm.org/show_bug.cgi?id=32706

Ie, we're not checking if the 'xor' is actually a 'not', so we could reverse
a 'not' optimization and cause an infinite loop by altering an 'xor X, -1'. 

Differential Revision: https://reviews.llvm.org/D33050

llvm-svn: 302733