Refactor a lot of patchpoint/stackmap related code to simplify and make it
target independent.

Most of the x86 specific stackmap/patchpoint handling was necessitated by the
use of the native address-mode format for frame index operands. PEI has now
been modified to treat stackmap/patchpoint similarly to DEBUG_INFO, allowing
us to use a simple, platform independent register/offset pair for frame
indexes on stackmap/patchpoints.

Notes:
  - Folding is now platform independent and automatically supported.
  - Emiting patchpoints with direct memory references now just involves calling
    the TargetLoweringBase::emitPatchPoint utility method from the target's
    XXXTargetLowering::EmitInstrWithCustomInserter method. (See
    X86TargetLowering for an example).
  - No more ugly platform-specific operand parsers.

This patch shouldn't change the generated output for X86.