[x86] don't blindly transform SETB into SBB

I noticed unnecessary 'sbb' instructions in D30472 and while looking at 'ptest' codegen recently. 
This happens because we were transforming any 'setb' - even when we only wanted a single-bit result.

This patch moves those transforms under visitAdd/visitSub, so we we're only creating sbb/adc when it
is a win. I don't know why we need a SETCC_CARRY node type, but I'm not proposing to change that
existing behavior in this patch.

Also, I'm skeptical that sbb/adc are a win for all micro-arches, so I added comments to the test files
where this transform still fires.

The test changes here are all cases where we no longer produce sbb/adc. Avoiding partial register
stalls (generating an xor to clear a register) is not handled in some cases, but that's a separate
issue.

Differential Revision: https://reviews.llvm.org/D30611

llvm-svn: 297586