Extend SimplifyCFG's common-destination folding heuristic to allow a single

Extend SimplifyCFG's common-destination folding heuristic to allow a single
"bonus" instruction to be speculatively executed.  Add a heuristic to
ensure we're not tripping up out-of-order execution by checking that this bonus
instruction only uses values that were already guaranteed to be available.

This allows us to eliminate the short circuit in (x&1)&&(x&2).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@108351 91177308-0d34-0410-b5e6-96231b3b80d8