[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