[Issue 24406] New: wrong signess of comparison with -O and byte-casted char

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24406

          Issue ID: 24406
           Summary: wrong signess of comparison with -O and byte-casted
                    char
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: a.horodniceanu at proton.me

Reduced from
https://github.com/Pure-D/serve-d/blob/master/lsp/source/served/lsp/textdocumentmanager.d#L1309:
```
void main () {
        char c = (() => cast(char)47)(); // c == '/'
        assert(cast(byte)c >= -0x40);
}
```

```
$ dmd -O -run main.d
core.exception.AssertError at main.d(3): Assertion failure
----------------
??:? _d_assertp [0x55635be3f818]
??:? _Dmain [0x55635be3f76b]
```
$ dmd -run main.d
<no error>
```

Tested against master.

The code generated is:
```
   0x0000555555597748 <+0>:     push   %rbp
   0x0000555555597749 <+1>:     mov    %rsp,%rbp
   0x000055555559774c <+4>:     rex.W call *0x4b7fd(%rip)        #
0x5555555e2f50
   0x0000555555597753 <+11>:    cmp    $0xc0,%al
   0x0000555555597755 <+13>:    jb     0x55555559775b <_Dmain+19>
   0x0000555555597757 <+15>:    xor    %eax,%eax
   0x0000555555597759 <+17>:    pop    %rbp
   0x000055555559775a <+18>:    ret
   0x000055555559775b <+19>:    mov    $0x3,%esi
   0x0000555555597760 <+24>:    lea    0x398b9(%rip),%rdi        #
0x5555555d1020 <_TMP0>
   0x0000555555597767 <+31>:    call   0x5555555977d0 <_d_assertp>
   0x000055555559776c <+36>:    pop    %rbp
   0x000055555559776d <+37>:    ret
```
The problem is with `jb 0x55555559775b <_Dmain+19>` since the comparison should
be signed.

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