[x86] add SBB optimization for SETBE (ule) 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. 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
rL307404 (D34652)

As acknowledged in the earlier review, there's a possibility that some Intel
uarch would prefer to produce an xor to clear the fake register operand with
sbb %eax, %eax. This will likely need to be addressed in a separate pass.

llvm-svn: 307471

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