[x86] add SBB optimization for SETAE (uge) condition code

x86 scalar select-of-constants (Cond ? C1 : C2) combining/lowering is a mess 
with missing optimizations. We handle some patterns, but miss logical variants.

To clean that up, we should convert all select-of-constants to logic/math and 
enhance the combining for the expected patterns from that. DAGCombiner already 
has the foundation to allow the transforms, so we just need to fill in the holes 
for x86 math op lowering. Selecting 0 or -1 needs extra attention to produce the
optimal code as shown here.

Attempt to verify that all of these IR forms are logically equivalent:
http://rise4fun.com/Alive/plxs

Earlier steps in this series:
rL306040
rL306072

Differential Revision: https://reviews.llvm.org/D34652

llvm-svn: 307404

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