This is a follow-up to the discussion in D12882.

Ideally, we would like SimplifyCFG to be able to form select instructions even when the operands
are expensive (as defined by the TTI cost model) because that may expose further optimizations. 
However, we would then like a later pass like CodeGenPrepare to undo that transformation if the
target would likely benefit from not speculatively executing an expensive op (this patch).

Once we have this safety mechanism in place, we can adjust SimplifyCFG to restore its 
select-formation behavior that changed with r248439.

Differential Revision: http://reviews.llvm.org/D13297

llvm-svn: 250527