Allow Integer Overflow

[rpkak]

This is also in netwide-assembler/nasm.

This fixes among other things executing ./zig-out/bin/nasm -fwin64 a.asm if compiled using Debug or ReleaseSafe (only tested on linux). Before this PR, this failed, because this shift overflows.

a.asm can be as simple as:

    section .text
    global _main
_main:
    add eax, ebx