[llvm] [RISCV][MRI] Account for fixed registers when determining callee saved regs (PR #115756)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> Does this change the stack layout of callee saved registers when one is reserved? 

This patch causes us not to spill the reserved register, when we were spilling it before. This by definition changes the stack layout.

> Does it match gcc stack layout?

I tried with this program with my patch applied:

```
register long a asm("x24");
int bar();
int baz(int a, int b, int c, int d);
int foo(int b, int c, int d) {
  a = 123;
  bar();
  baz(a, b, c, d);
  return 0;
}
```

GCC:
```
sd      ra,24(sp)
sd      s0,16(sp)
sd      s1,8(sp)
sd      s2,0(sp)
```
LLVM + this patch:
```
sd ra, 24(sp) # 8-byte Folded Spill
sd s0, 16(sp) # 8-byte Folded Spill
sd s1, 8(sp) # 8-byte Folded Spill
sd s2, 0(sp) # 8-byte Folded Spill
```

Looks the same on this example.

https://github.com/llvm/llvm-project/pull/115756