AArch64: add support for llvm.aarch64.hint intrinsic

This adds a llvm.aarch64.hint intrinsic to mirror the llvm.arm.hint in order to
support the various hint intrinsic functions in the ACLE.

Add an optional pattern field that permits the subclass to specify the pattern
that matches the selection.  The intrinsic pattern is set as mayLoad, mayStore,
so overload the value for the definition of the hint instruction.