Re: [quicwg/base-drafts] Change packet types (#1655)

[MikkelFJ <[e-mail]>]

`((int8_t)pkt_type + (int8_t)0x40) < 0`

The add operation should overflow and set the sign if the two upper bits are set and not in any other case. clang x64 does it slightly different using a cmp 0x40 followed by a conditional set in the example below, but that could be a conditional jump. 

```
#include <stdint.h>
#include <stdio.h>

int test(uint8_t pkt_type)
{
    int8_t x = (int8_t)pkt_type;
    return (x + (int8_t)0x40) < 0;
}
```
clang -c -S -O3 test.c

```
    .section    __TEXT,__text,regular,pure_instructions
    .macosx_version_min 10, 13
    .globl  _test                   ## -- Begin function test
    .p2align    4, 0x90
_test:                                  ## @test
    .cfi_startproc
## BB#0:
    pushq   %rbp
Lcfi0:
    .cfi_def_cfa_offset 16
Lcfi1:
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
Lcfi2:
    .cfi_def_cfa_register %rbp
    xorl    %eax, %eax
    cmpb    $-64, %dil
    setl    %al
    popq    %rbp
    retq
    .cfi_endproc
                                        ## -- End function

.subsections_via_symbols
```

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