[x86] add/sub (X==0) --> sbb(cmp X, 1)

This is very similar to the transform in:
https://reviews.llvm.org/rL306040
...but in this case, we use cmp X, 1 to set the carry bit as needed.

Again, we can show that all of these are logically equivalent (although
InstCombine currently canonicalizes to a form not seen here), and if
we believe IACA, then this is the smallest/fastest code. Eg, with SNB:

| Num Of |              Ports pressure in cycles               |    |
|  Uops  |  0  - DV  |  1  |  2  -  D  |  3  -  D  |  4  |  5  |    |
---------------------------------------------------------------------
|   1    | 1.0       |     |           |           |     |     |    | cmp edi, 0x1
|   2    |           | 1.0 |           |           |     | 1.0 | CP | sbb eax, eax


The larger motivation is to clean up all select-of-constants combining/lowering 
because we're missing some common cases.

llvm-svn: 306072

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