[llvm] support save/restore point splitting in shrink-wrap

This patch introduces "-enable-shrink-wrap-into-multiple-points"
option, which enables splitting Save and Restore points during ShrinkWrap pass, i.e.
insert registers saves and restores as close as possible to their usage.

Current algorithm disables Save / Restore point splitting for
functions with instructions with FrameIndex operands,
with EHPads and with any Stack accesses beacuse it is difficult to prove the safety of it.

This patch also add support for multiple Save / Restore points only for RISCV.

Now ShrinkWrap produces:
- list of SavePoint + Registers
- list of RestorePoint + Registers
- Prolog (NCD of Save points)
- Epilog (NCPD of Restore points)

Author: enoskova-sc

llvm/include/llvm/CodeGen/TargetFrameLowering.h

@@ -199,6 +199,10 @@ class TargetFrameLowering {
     return false;
   }
 
+  /// enableCSRSaveRestorePointsSplit - Returns true if the target support
+  /// multiple save/restore points in shrink wrapping.
+  virtual bool enableCSRSaveRestorePointsSplit() const { return false; }
+
   /// Returns true if the stack slot holes in the fixed and callee-save stack
   /// area should be used when allocating other stack locations to reduce stack
   /// size.