[CodeGen] Add a pass to do block predication on SSA machine IR.

    For targets requiring aggressive scheduling and/or software pipeline we need to
    apply predication before preRA scheduling. This adds a pass re-using the early
    if-cvt infrastructure but generating predicated instructions instead of
    speculatively executing instructions. It allows doing if conversion on blocks
    containing instructions with side-effects. The pass re-use the target hook from
    postRA if-conversion to let the target decide on the heuristic to apply.

    Differential Revision: https://reviews.llvm.org/D66190

llvm-svn: 369395