[AVR] Cherry-pick (D68524) Rewrite the function calling convention.

This fixes avr-rust/rust-legacy-fork#92.

At the time of cherry-picking, this patch has not yet been upstreamed.

Author: dylanmckay

llvm/lib/Target/AVR/AVRCallingConv.td

@@ -6,21 +6,13 @@
 //
 //===----------------------------------------------------------------------===//
 // This describes the calling conventions for AVR architecture.
+// Normal functions use a special calling convention, solved in code.
 //===----------------------------------------------------------------------===//
 
 //===----------------------------------------------------------------------===//
 // AVR Return Value Calling Convention
 //===----------------------------------------------------------------------===//
 
-def RetCC_AVR : CallingConv
-<[
-  // i8 is returned in R24.
-  CCIfType<[i8], CCAssignToReg<[R24]>>,
-
-  // i16 are returned in R25:R24, R23:R22, R21:R20 and R19:R18.
-  CCIfType<[i16], CCAssignToReg<[R25R24, R23R22, R21R20, R19R18]>>
-]>;
-
 // Special return value calling convention for runtime functions.
 def RetCC_AVR_BUILTIN : CallingConv
 <[
@@ -41,14 +33,6 @@ def ArgCC_AVR_Vararg : CallingConv
   CCAssignToStack<2, 1>
 ]>;
 
-// Special argument calling convention for
-// division runtime functions.
-def ArgCC_AVR_BUILTIN_DIV : CallingConv
-<[
-  CCIfType<[i8], CCAssignToReg<[R24,R22]>>,
-  CCIfType<[i16], CCAssignToReg<[R25R24, R23R22]>>
-]>;
-
 //===----------------------------------------------------------------------===//
 // Callee-saved register lists.
 //===----------------------------------------------------------------------===//