[rustc_span][perf] Hoist lookup sorted by words out of the loop.
@lqd commented on https://github.com/rust-lang/rust/pull/114351 asking
if `sort_by_words(lookup)` is computed repeatedly. I was assuming that
rustc should have no difficulties to hoist it automatically outside of
the loop to avoid repeated pure computation, but according to
https://godbolt.org/z/frs8Kj1rq it seems like I was wrong:
original version seems to have 2 calls per loop iteration
```
.LBB16_3:
mov rbx, qword ptr [r13]
mov r14, qword ptr [r13 + 8]
lea rdi, [rsp + 40]
mov rsi, rbx
mov rdx, r14
call example::sort_by_words
lea rdi, [rsp + 64]
mov rsi, qword ptr [rsp + 8]
mov rdx, qword ptr [rsp + 16]
call example::sort_by_words
mov rdi, qword ptr [rsp + 40]
mov rdx, qword ptr [rsp + 56]
mov rsi, qword ptr [rsp + 64]
cmp rdx, qword ptr [rsp + 80]
mov qword ptr [rsp + 32], rdi
mov qword ptr [rsp + 24], rsi
jne .LBB16_5
call qword ptr [rip + bcmp@GOTPCREL]
test eax, eax
sete al
mov dword ptr [rsp + 4], eax
mov rsi, qword ptr [rsp + 72]
test rsi, rsi
jne .LBB16_8
jmp .LBB16_9
```
but the manually hoisted version just 1:
```
.LBB16_3:
mov r13, qword ptr [r15]
mov r14, qword ptr [r15 + 8]
lea rdi, [rsp + 64]
mov rsi, r13
mov rdx, r14
call example::sort_by_words
mov rdi, qword ptr [rsp + 64]
mov rdx, qword ptr [rsp + 16]
cmp qword ptr [rsp + 80], rdx
mov qword ptr [rsp + 32], rdi
jne .LBB16_5
mov rsi, qword ptr [rsp + 8]
call qword ptr [rip + bcmp@GOTPCREL]
test eax, eax
sete bpl
mov rsi, qword ptr [rsp + 72]
test rsi, rsi
jne .LBB16_8
jmp .LBB16_9
```
This code is probably not very hot, but there is no reason to leave
such a low hanging fruit.
6ae2677d72