[MachineBasicBlock] Add helpers for skipping debug instructions [1/10]

[vsk]

Summary

These helpers are exercised by follow-up commits in this patch series,
which is all about removing CodeGen differences with vs. without debug
info in the AArch64 backend.

[fhahn]

LGTM, the set of patches to use the new utils shows that those utilities can be quite useful. Please wait a day or so with committing though, in case there are additional suggestions.

llvm/include/llvm/CodeGen/MachineBasicBlock.h

1067

It looks like some existing uses of skipDebugInstructionsForward could be replaced by next_nodbg().

1073

It looks like some existing uses of skipDebugInstructionsBackward could be replaced by prev_nodbg().

[vsk]

Fold in changes to use {prev,next}_nodbg where they provide a clear readability
improvement.

llvm/include/llvm/CodeGen/MachineBasicBlock.h

 /// const_instr_iterator} and the respective reverse iterators.
 template<class IterT>
 inline IterT skipDebugInstructionsBackward(IterT It, IterT Begin) {
   while (It != Begin && It->isDebugInstr())
     It--;
   return It;
 }
 
+/// Increment \p It, then continue incrementing it while it points to a debug
+/// instruction. A replacement for std::next.
+template <typename IterT> inline IterT next_nodbg(IterT It, IterT End) {
+  return skipDebugInstructionsForward(std::next(It), End);
+}
+
+/// Decrement \p It, then continue decrementing it while it points to a debug
+/// instruction. A replacement for std::prev.
+template <typename IterT> inline IterT prev_nodbg(IterT It, IterT Begin) {
+  return skipDebugInstructionsBackward(std::prev(It), Begin);
+}
+
+/// Construct a range iterator which begins at \p It and moves forwards until
+/// \p End is reached, skipping any debug instructions.
+template <typename IterT>
+inline auto instructionsWithoutDebug(IterT It, IterT End) {
+  return make_filter_range(make_range(It, End), [](const MachineInstr &MI) {
+    return !MI.isDebugInstr();
+  });
+}
+
+/// Construct a range iterator which begins at \p It and moves backwards until
+/// \p Begin is reached, skipping any debug instructions.
+template <typename IterT>
+inline auto reversedInstructionsWithoutDebug(IterT It, IterT Begin) {
+  return instructionsWithoutDebug(make_reverse_iterator(It),
+                                  make_reverse_iterator(Begin));
+}
+
 } // end namespace llvm
 
 #endif // LLVM_CODEGEN_MACHINEBASICBLOCK_H