[x86] add/sub (X==0) --> sbb(neg X)

Our handling of select-of-constants is lumpy in IR (https://reviews.llvm.org/D24480),
lumpy in DAGCombiner, and lumpy in X86ISelLowering. That's why we only had the 'sbb'
codegen in 1 out of the 4 tests. This is a step towards smoothing that out.

First, show that all of these IR forms are equivalent:
http://rise4fun.com/Alive/mx

Second, show that the 'sbb' version is faster/smaller. IACA output for SandyBridge
(later Intel and AMD chips are similar based on Agner's tables):

This is the "obvious" x86 codegen (what gcc appears to produce currently):

| Num Of |              Ports pressure in cycles               |    |
|  Uops  |  0  - DV  |  1  |  2  -  D  |  3  -  D  |  4  |  5  |    |
---------------------------------------------------------------------
|   1*   |           |     |           |           |     |     |    | xor eax, eax
|   1    | 1.0       |     |           |           |     |     | CP | test edi, edi
|   1    |           |     |           |           |     | 1.0 | CP | setnz al
|   1    |           | 1.0 |           |           |     |     | CP | neg eax


This is the adc version:
|   1*   |           |     |           |           |     |     |    | xor eax, eax
|   1    | 1.0       |     |           |           |     |     | CP | cmp edi, 0x1
|   2    |           | 1.0 |           |           |     | 1.0 | CP | adc eax, 0xffffffff


And this is sbb:
|   1    | 1.0       |     |           |           |     |     |    | neg edi
|   2    |           | 1.0 |           |           |     | 1.0 | CP | sbb eax, eax

If IACA is trustworthy, then sbb became a single uop in Broadwell, so this will be
clearly better than the alternatives going forward.

llvm-svn: 306040

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