[AArch64] Simplify aarch64_can_eliminate

Simplify aarch64_can_eliminate - if we need a frame pointer, we must
eliminate to HARD_FRAME_POINTER_REGNUM.  Rather than hardcoding all
combinations from the ELIMINABLE_REGS list, just do the correct check.

    gcc/
	* config/aarch64/aarch64.c (aarch64_can_eliminate): Simplify logic.

From-SVN: r254534

gcc/ChangeLog

 2017-11-08  Wilco Dijkstra  <wdijkstr@arm.com>
 
+	* config/aarch64/aarch64.c (aarch64_can_eliminate): Simplify logic.
+
+2017-11-08  Wilco Dijkstra  <wdijkstr@arm.com>
+
 	* config/aarch64/aarch64.c (aarch64_frame_pointer_required)
 	Remove.
 	(aarch64_layout_frame): Initialise emit_frame_chain.

gcc/config/aarch64/aarch64.c

@@ -5913,26 +5913,14 @@ aarch64_secondary_reload (bool in_p ATTRIBUTE_UNUSED, rtx x,
 }
 
 static bool
-aarch64_can_eliminate (const int from, const int to)
+aarch64_can_eliminate (const int from ATTRIBUTE_UNUSED, const int to)
 {
-  /* If we need a frame pointer, we must eliminate FRAME_POINTER_REGNUM into
-     HARD_FRAME_POINTER_REGNUM and not into STACK_POINTER_REGNUM.  */
+  gcc_assert (from == ARG_POINTER_REGNUM || from == FRAME_POINTER_REGNUM);
 
+  /* If we need a frame pointer, ARG_POINTER_REGNUM and FRAME_POINTER_REGNUM
+     can only eliminate to HARD_FRAME_POINTER_REGNUM.  */
   if (frame_pointer_needed)
-    {
-      if (from == ARG_POINTER_REGNUM && to == HARD_FRAME_POINTER_REGNUM)
-	return true;
-      if (from == ARG_POINTER_REGNUM && to == STACK_POINTER_REGNUM)
-	return false;
-      if (from == FRAME_POINTER_REGNUM && to == STACK_POINTER_REGNUM
-	  && !cfun->calls_alloca)
-	return true;
-      if (from == FRAME_POINTER_REGNUM && to == HARD_FRAME_POINTER_REGNUM)
-	return true;
-
-      return false;
-    }
-
+    return to == HARD_FRAME_POINTER_REGNUM;
   return true;
 }