[InstCombine] move icmp bool canonicalizations to helper; NFC
[InstCombine] move icmp bool canonicalizations to helper; NFC

As noted in the post-commit comments in D32143, we should be
catching the constant operand cases sooner to be more efficient
and less likely to expose a missing fold.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@303309 91177308-0d34-0410-b5e6-96231b3b80d8

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