[llvm] [AArch64] Separate PNR into its own Register Class (PR #65306)

[Sander de Smalen via llvm-commits]

================
@@ -80,8 +80,8 @@ body:             |
     ; CHECK-NEXT:     stack-id: scalable-vector, callee-saved-register: ''
 
     ; EXPAND-LABEL: name: spills_fills_stack_id_pnr
-    ; EXPAND: STR_PXI $pn0, $sp, 7
-    ; EXPAND: $pn0 = LDR_PXI $sp, 7
+    ; EXPAND: STR_PXI $p0, $sp, 7, implicit-def $pn0
----------------
sdesmalen-arm wrote:

This should also not implicitly define `$pn0`, because no registers are defined by a store.

https://github.com/llvm/llvm-project/pull/65306