[Stackmaps] Improve the stackmap lowering code in the SelectionDAGBuilder.

We are now no longer relying on the target-specific call lowering implementation
to lower a stackmap intrinsic call. Instead we perform the call lowering in a
target-independent way directly in the stackmap lowering code. This simplifies
the code and removes the need to fixup the code after the target-specific call
lowering.

llvm-svn: 201263