Disable shrink-wrap for Thumb1

[weimingz]

Summary

In ARMConstantIslandPass, which runs after Shrink Wrap pass, long jumps will be fixed up as BL (tBfar) which depends on spilling LR in epilogue. However, shrink-wrap may remove the LR, which causes issues when the function returns.

[rengolin]

Shrink wrap support in Thumb is not very well tested anyway, I think it's safe to disable until proven stable.

lib/Target/ARM/Thumb1FrameLowering.h

public:
 
   /// Check whether or not the given \p MBB can be used as a epilogue
   /// for the target.
   /// The epilogue will be inserted before the first terminator of that block.
   /// This method is used by the shrink-wrapping pass to decide if
   /// \p MBB will be correctly handled by the target.
   bool canUseAsEpilogue(const MachineBasicBlock &MBB) const override;
 
+  /// Disable shrink wrap as tBfar/BL will be used to adjust for long jumps.
+  bool enableShrinkWrapping(const MachineFunction &MF) const override {
+    return false;
+  }
+
 private:
   /// Check if the frame lowering of \p MF needs a special fixup
   /// code sequence for the epilogue.
   /// Unlike T2 and ARM mode, the T1 pop instruction cannot restore
   /// to LR, and we can't pop the value directly to the PC when
   /// we need to update the SP after popping the value. So instead
   /// we have to emit:
   ///   POP {r3}