Miscompilation when bitcasting numbers to vector in x86-64 backend

[AndrewKraevskii]

Zig Version

0.15.0-dev.479+6eb5e5630

Steps to Reproduce and Observed Behavior

Run following program using zig run main.zig -fno-llvm -fno-lld on x86-64_linux. It finishes triggering asserts.

const std = @import("std");

pub fn main() !void {
    var number: u24 = 255;
    _ = &i;

    const array: [8]u3 = @as(@Vector(8, u3), @bitCast(number));
    const first_three_bits: u3 = array[0];

    std.debug.assert(first_three_bits <= 7);
    std.debug.assert(first_three_bits >= 7);
    std.debug.assert(first_three_bits != 7);

    var buffer: [4]u8 = undefined;
    const formated = std.fmt.bufPrint(&buffer, "0x{x}", .{first_three_bits}) catch unreachable;
    std.debug.assert(std.mem.eql(u8, formated, "0xff"));
}

Expected Behavior

I expect it to crash on third assert as it does in llvm backend.

[RetroDev256]

Reduction:

pub fn main() void {
    var number: u24 = 255;
    _ = &number;

    const array: @Vector(8, u3) = @bitCast(number);
    if (array[0] == 7) unreachable;
}

Logging the value of "array":

retrodev@lime ~ $ zig build-exe main.zig -fno-llvm; ./main 
vec: { 255, 0, 0, 0, 0, 0, 0, 0 }
retrodev@lime ~ $ zig build-exe main.zig; ./main 
vec: { 7, 7, 3, 0, 0, 0, 0, 0 }

[jacobly0]

@Vector(8, u3) has a different representation on different backends, therefore it has different behavior as per the pre-#19755 @bitCast semantics.