[AArch64] Stop reserved registers from being saved in prolog/epilog (#138448)

[GCC's
documentation](https://gcc.gnu.org/onlinedocs/gcc-15.1.0/gcc/Code-Gen-Options.html)
is clear on how -ffixed-reg must behave:
```
  Treat the register named reg as a fixed register; generated
  code should never refer to it (except perhaps as a stack pointer,
  frame pointer or in some other fixed role).
```

This implies prolog/epilog code also must not save/restore explicitly
fixed registers, even when it is callee-saved. Some projects rely on
this (GCC's) behavior.

For example,
```
void f() {
  register uint64_t x28 asm("x28") = 0xee;
  asm volatile("" : "+r"(x28)); // avoid mov being eliminated
}
```
should not touch x28 outside of `mov w28,#0xee`.

For riscv64, clang behaves the same as GCC, so I am inclined to believe
this is indeed a bug.

Fixes #111379.

0 files changed, 0 insertions, 0 deletions