[X86] Failure to shift-in one bits via `shld`

[Zig Godbolt](https://zig.godbo.lt/#g:!((g:!((g:!((h:codeEditor,i:(filename:'1',fontScale:14,fontUsePx:'0',j:1,lang:zig,selection:(endColumn:2,endLineNumber:11,positionColumn:1,positionLineNumber:5,selectionStartColumn:2,selectionStartLineNumber:11,startColumn:1,startLineNumber:5),source:'export+fn+shift_in_ones(a:+u64,+c:+u8)+u64+%7B%0A++++return+~(~a+%3C%3C+@truncate(c))%3B%0A%7D%0A%0Aexport+fn+shld(a:+u64,+b:+u64,+c:+u8)+u64+%7B%0A++++return+@truncate(@as(u128,+@bitCast(%5B2%5Du64%7B+a,+b+%7D))+%3C%3C+@as(u6,+@truncate(c))+%3E%3E+64)%3B%0A%7D%0A%0Aexport+fn+shift_in_ones_2(a:+u64,+c:+u8)+u64+%7B%0A++++return+shld(~@as(u64,+0),+a,+c)%3B%0A%7D%0A%0Aexport+fn+shift_in_ones_3(a:+u64,+c:+u8)+u64+%7B%0A++++return+shld_asm(~@as(u64,+0),+a,+c)%3B%0A%7D%0A%0Apub+fn+shld_asm(low:+u64,+high:+u64,+shift_amt:+u8)+u64+%7B%0A++++var+ret:+u64+%3D+high%3B%0A++++asm+(%22shldq+%25%5Bcnt%5D,+%25%5Bsrc%5D,+%25%5Bret%5D%22%0A++++++++:+%5Bret%5D+%22%2Br%22+(ret),%0A++++++++:+%5Bsrc%5D+%22r%22+(low),%0A++++++++++%5Bcnt%5D+%22%7Bcl%7D%22+(shift_amt),%0A++++++++:+.%7B+.cc+%3D+true+%7D)%3B%0A++++return+ret%3B%0A%7D'),l:'5',n:'0',o:'Zig+source+%231',t:'0')),k:55.575874538014034,l:'4',m:100,n:'0',o:'',s:0,t:'0'),(g:!((h:compiler,i:(compiler:ztrunk,filters:(b:'0',binary:'1',binaryObject:'1',commentOnly:'0',debugCalls:'1',demangle:'0',directives:'0',execute:'1',intel:'0',libraryCode:'0',trim:'1',verboseDemangling:'0'),flagsViewOpen:'1',fontScale:14,fontUsePx:'0',j:2,lang:zig,libs:!(),options:'-O+ReleaseFast+-target+x86_64-linux+-mcpu%3Dicelake_server+-fomit-frame-pointer',overrides:!(),selection:(endColumn:12,endLineNumber:27,positionColumn:1,positionLineNumber:21,selectionStartColumn:12,selectionStartLineNumber:27,startColumn:1,startLineNumber:21),source:1),l:'5',n:'0',o:'+zig+trunk+(Editor+%231)',t:'0')),header:(),k:44.42412546198598,l:'4',m:100.00000000000001,n:'0',o:'',s:0,t:'0')),l:'2',m:100,n:'0',o:'',t:'0')),version:4)

Attempt 1:

```zig
export fn shift_in_ones(a: u64, c: u8) u64 {
return ~(~a << @truncate(c));
}
```

```asm
shift_in_ones:
not rdi
shlx rax, rdi, rsi
not rax
ret
```

Attempt 2:

```zig
export fn shld(a: u64, b: u64, c: u8) u64 {
return @truncate(@as(u128, @bitCast([2]u64{ a, b })) << @as(u6, @truncate(c)) >> 64);
}

export fn shift_in_ones_2(a: u64, c: u8) u64 {
return shld(~@as(u64, 0), a, c);
}
```

```asm
shld:
mov ecx, edx
mov rax, rsi
shld rax, rdi, cl
ret

shift_in_ones_2: ; for some reason inlining makes us not see that this a `shld`?
shlx rcx, rdi, rsi
and sil, 63
not sil
movabs rax, 9223372036854775807
shrx rax, rax, rsi
or rax, rcx
ret
```

Should be:

```asm
shift_in_ones:
mov ecx, esi
mov rax, rdi
mov rdx, -1
shld rax, rdx, cl
ret
```

Start with the Zig Godbolt reproducer and compare the generated assembly for shift_in_ones, shld, and shift_in_ones_2 against the expected shld sequence shown in the issue. Trace the compiler's handling of the inlined shift-in-ones pattern; done means the relevant function recognizes the pattern and emits the expected x86 shld form.

Written by the indexing model from the issue text.