[llvm] [AArch64][PAC][GISel] Add missing clobbering info to LOADgotAUTH (PR #157433)

[Anatoly Trosinenko via llvm-commits]

atrosinenko wrote:

Here is an example of incorrect code generation:
```c
extern unsigned long pac_field_mask;
static void f() {}

unsigned long get_non_signed_function_pointer(void) {
  return ((unsigned long)f) & ~pac_field_mask;
}
```

Compiled at `-O0` optimization level with `clang -target aarch64-linux-pauthtest -march=v8.3-a -Xclang -fptrauth-elf-got -S -o- example.c`:

```
get_non_signed_function_pointer:        // @get_non_signed_function_pointer
	.cfi_startproc
// %bb.0:                               // %entry
                                        // implicit-def: $x16
                                        // implicit-def: $x17
	adrp	x16, f
	add	x16, x16, :lo12:f
	paciza	x16
	adrp	x17, :got_auth:pac_field_mask
	add	x17, x17, :got_auth_lo12:pac_field_mask
	ldr	x16, [x17]
	autda	x16, x17
	mov	x17, x16
	xpacd	x17
	cmp	x16, x17
	b.eq	.Lauth_success_0
	brk	#0xc472
.Lauth_success_0:
	mov	x8, x16
	ldr	x8, [x8]
	bic	x0, x16, x8
	ret
```

<details>
<summary>With this patch applied</summary>

```
get_non_signed_function_pointer:        // @get_non_signed_function_pointer
	.cfi_startproc
// %bb.0:                               // %entry
	sub	sp, sp, #16
	.cfi_def_cfa_offset 16
                                        // implicit-def: $x16
                                        // implicit-def: $x17
	adrp	x16, f
	add	x16, x16, :lo12:f
	paciza	x16
	str	x16, [sp, #8]                   // 8-byte Folded Spill
	adrp	x17, :got_auth:pac_field_mask
	add	x17, x17, :got_auth_lo12:pac_field_mask
	ldr	x16, [x17]
	autda	x16, x17
	mov	x17, x16
	xpacd	x17
	cmp	x16, x17
	b.eq	.Lauth_success_0
	brk	#0xc472
.Lauth_success_0:
	mov	x8, x16
	ldr	x16, [sp, #8]                   // 8-byte Folded Reload
	ldr	x8, [x8]
	bic	x0, x16, x8
	add	sp, sp, #16
	.cfi_def_cfa_offset 0
	ret
```

</details>

Compiled with `-fglobal-isel` with `clang -target aarch64-linux-pauthtest -march=v8.3-a -Xclang -fptrauth-elf-got -S -o- example.c -O1 -fglobal-isel`:

```
get_non_signed_function_pointer:        // @get_non_signed_function_pointer
	.cfi_startproc
// %bb.0:                               // %entry
	adrp	x16, f
	add	x16, x16, :lo12:f
	paciza	x16
	adrp	x17, :got_auth:pac_field_mask
	add	x17, x17, :got_auth_lo12:pac_field_mask
	ldr	x16, [x17]
	autda	x16, x17
	mov	x17, x16
	xpacd	x17
	cmp	x16, x17
	b.eq	.Lauth_success_0
	brk	#0xc472
.Lauth_success_0:
	mov	x8, x16
	ldr	x8, [x8]
	bic	x0, x16, x8
	ret
```

<details>
<summary>With this patch applied</summary>

```
get_non_signed_function_pointer:        // @get_non_signed_function_pointer
	.cfi_startproc
// %bb.0:                               // %entry
	adrp	x16, f
	add	x16, x16, :lo12:f
	paciza	x16
	mov	x8, x16
	adrp	x17, :got_auth:pac_field_mask
	add	x17, x17, :got_auth_lo12:pac_field_mask
	ldr	x16, [x17]
	autda	x16, x17
	mov	x17, x16
	xpacd	x17
	cmp	x16, x17
	b.eq	.Lauth_success_0
	brk	#0xc472
.Lauth_success_0:
	mov	x9, x16
	ldr	x9, [x9]
	bic	x0, x8, x9
	ret
```

</details>

https://github.com/llvm/llvm-project/pull/157433