[EarlyIfConversion] Add target hook to allow for multiple ifcvt iterations.

Adds a target hook canPredicatePredicatedInstr(const MachineInstr&) that
assumes an instruction is already predicated and returns true if it can
be predicated again, used by the early if-conversion pass in order to
iterate multiple times on architectures supporting predicate logic.

No test added since there is no upstream target that can take advantage.

Differential Revision: https://reviews.llvm.org/D139981