[CGP] transform select instructions into branches and sink expensive operands

Description

[CGP] transform select instructions into branches and sink expensive operands

This was originally checked in at r250527, but reverted at r250570 because of PR25222.
There were at least 2 problems:

  1. The cost check was checking for an instruction with an exact cost of TCC_Expensive;

that should have been >=.

  1. The cause of the clang stage 1 failures was illegally sinking 'call' instructions;

we can't sink instructions that may have side effects / are not safe to execute speculatively.

Fixed those conditions in sinkSelectOperand() and added test cases.

Original commit message:
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