[X86] Optimize stackmap shadows on X86.

This patch minimizes the number of nops that must be emitted on X86 to satisfy
stackmap shadow constraints.

To minimize the number of nops inserted, the X86AsmPrinter now records the
size of the most recent stackmap's shadow in the StackMapShadowTracker class,
and tracks the number of instruction bytes emitted since the that stackmap
instruction was encountered. Padding is emitted (if it is required at all)
immediately before the next stackmap/patchpoint instruction, or at the end of
the basic block.

This optimization should reduce code-size and improve performance for people
using the llvm stackmap intrinsic on X86.

<rdar://problem/14959522>



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213892 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/AsmPrinter.h

@@ -264,6 +264,9 @@ public:
   /// function.
   virtual void EmitFunctionBodyEnd() {}
 
+  /// Targets can override this to emit stuff at the end of a basic block.
+  virtual void EmitBasicBlockEnd(const MachineBasicBlock &MBB) {}
+
   /// Targets should implement this to emit instructions.
   virtual void EmitInstruction(const MachineInstr *) {
     llvm_unreachable("EmitInstruction not implemented");