[NFC][Codegen] Add better test coverage for potential add/sub constant folding

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@362110 91177308-0d34-0410-b5e6-96231b3b80d8