The computed value is only used when the register is equal to zero, so
we can fully precompute it and materialize the constant instead. In
other words, we change from
```
return reg == 0 ? (reg | 1ULL << 63) : reg;
```
to
```
return reg == 0 ? 1ULL << 63 : reg;
```
The number of instructions remains the same, but we eliminate an
unnecessary dependency on the register value.
Before:
0xb241037a orr x26, x27, #0x8000000000000000
0xeb1f037f cmp x27, xzr
0x9a9a137b csel x27, x27, x26, ne
After:
0xd2f0001a mov x26, #-0x8000000000000000 ; =-9223372036854775808
0xeb1f037f cmp x27, xzr
0x9a9a137b csel x27, x27, x26, ne
24f2981e54