Standard way to supply hints to branches

[Walter Bright]

Compile the following with -vasm -O:

```
void bar();

int foo(int i)
{
     if (i)
         return 0;
     bar();
     return 1;
}

int baz(int i)
{
     if (i)
         goto Lreturn0;
     bar();
     return 1;

Lreturn0:
     return 0;
}
```
and you get:
```
_D5test93fooFiZi:
0000:   55                       push      RBP
0001:   48 8B EC                 mov       RBP,RSP
0004:   85 FF                    test      EDI,EDI
0006:   74 04                    je        Lc
0008:   31 C0                    xor       EAX,EAX // hot path
000a:   5D                       pop       RBP
000b:   C3                       ret
000c:   E8 00 00 00 00           call      L0   // cold path
0011:   B8 01 00 00 00           mov       EAX,1
0016:   5D                       pop       RBP
0017:   C3                       ret
_D5test93bazFiZi:
0000:   55                       push      RBP
0001:   48 8B EC                 mov       RBP,RSP
0004:   85 FF                    test      EDI,EDI
0006:   75 0C                    jne       L14
0008:   E8 00 00 00 00           call      L0   // hot path
000d:   B8 01 00 00 00           mov       EAX,1
0012:   5D                       pop       RBP
0013:   C3                       ret
0014:   31 C0                    xor       EAX,EAX // cold path
0016:   5D                       pop       RBP
0017:   C3                       ret
```