VASM part 6 continued: remove useless control flow

Summary: Enable vasm classic control flow optimizations. The actual implementation
is in D1494657 just not used yet; see vasm-jumps.cpp. This diff starts using it.

 * if both targets of a jcc are the same, convert to jmp
 * if an edge goes to a jmp, use the jmp's target
 * if B1 ends with a jmp to B2 and B2 has 1 pred, merge B1 & B2
 * if B1 ends with a jump to an empty block with a jcc, copy the jcc.

Reviewed By: @alexmalyshev

Differential Revision: D1460505
Fix external build?

Summary: One external report that this unused variable was causing an
error in the build. Our internal build went fine, so this might be a
change in default warnings between 3.12 and 4.01? In any event, easy
enough to fix, and also remove warn-error in the external build for now.

Reviewed By: @gabelevi

Differential Revision: D1512305