JitArm64_SystemRegister: mfspr - Sign extension nit

This instruction effectively performs the following computation:

```
Xresult = SXTW(slice_length) - Xresult + global_timer;
```

ARM64 has a dedicated sign extension instruction, but also allows this
sign extension to happen as part of many arithmetic operations. By
reordering the computation, we can take advantage of this:

```
Xresult = global_timer + SXTW(slice_length) - Xresult;
```

Before:
```
ldr    w24, [x29, #0x2f0]
scvtf  s8, w24
ldr    s9, [x27, #0x20]
fmul   s8, s8, s9
fcvtzs x25, s8
ldp    x24, x23, [x27]
sxtw   x23, w23
sub    x25, x23, x25
add    x25, x25, x24
ldp    x24, x23, [x27, #0x10]
sub    x25, x25, x23
mov    x23, #-0x5555555555555556 ; =-6148914691236517206
add    x23, x23, #0x1
umulh  x25, x25, x23
add    x25, x24, x25, lsr #3
str    x25, [x29, #0x770]
```

After:
```
ldr    w24, [x29, #0x2f0]
scvtf  s8, w24
ldr    s9, [x27, #0x20]
fmul   s8, s8, s9
fcvtzs x25, s8
ldp    x24, x23, [x27]
add    x24, x24, w23, sxtw
sub    x25, x24, x25
ldp    x24, x23, [x27, #0x10]
sub    x25, x25, x23
mov    x23, #-0x5555555555555556 ; =-6148914691236517206
add    x23, x23, #0x1
umulh  x25, x25, x23
add    x25, x24, x25, lsr #3
str    x25, [x29, #0x770]
```

0 files changed, 0 insertions, 0 deletions