[x86] Fix an amazing goof in the handling of sub, or, and xor lowering.

The comment for this code indicated that it should work similar to our
handling of add lowering above: if we see uses of an instruction other
than flag usage and store usage, it tries to avoid the specialized
X86ISD::* nodes that are designed for flag+op modeling and emits an
explicit test.

Problem is, only the add case actually did this. In all the other cases,
the logic was incomplete and inverted. Any time the value was used by
a store, we bailed on the specialized X86ISD node. All of this appears
to have been historical where we had different logic here. =/

Turns out, we have quite a few patterns designed around these nodes. We
should actually form them. I fixed the code to match what we do for add,
and it has quite a positive effect just within some of our test cases.
The only thing close to a regression I see is using:

  notl %r
  testl %r, %r

instead of:

  xorl -1, %r

But we can add a pattern or something to fold that back out. The
improvements seem more than worth this.

I've also worked with Craig to update the comments to no longer be
actively contradicted by the code. =[ Some of this still remains
a mystery to both Craig and myself, but this seems like a large step in
the direction of consistency and slightly more accurate comments.

Many thanks to Craig for help figuring out this nasty stuff.

Differential Revision: https://reviews.llvm.org/D37096

llvm-svn: 311737

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