[NFC][Codegen] Add better test coverage for potential add/sub constant folding
This adds hopefully-full test coverage for all the possible permutations:
First op is one of:
- x + c1
- x - c1
- c1 - x
Second op is one of:
- + c2
- - c2
- c2 -
And thus 3*3=9 patterns.
Some of them show missed constant-folds.
Without previous patch (the revert), these tests were causing endless
dagcombine loop. I really should have thought about this first :S