[NFC][InstCombine] Some tests for sub-of-negatible pattern
[NFC][InstCombine] Some tests for sub-of-negatible pattern

As we have previously estabilished, `sub` is an outcast,
and should be considered non-canonical iff it can be converted to `add`.

It can be converted to `add` if it's second operand can be negated.
So far we mostly only do that for constants and negation itself,
but we should be more through.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@373597 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm