`sizeOf(T) > 0` inconsistent result

Similar to #4527 but not quite the same? alignOf doesn't have this problem.

const std = @import("std");

pub fn main() void {
    std.debug.warn("immediate > 0: {}\n", .{ @sizeOf(*u0) > 0 });
    std.debug.warn("immediate > 0.01: {}\n", .{ @sizeOf(*u0) > 0.01 });

    f(*u0);
}

pub fn f(comptime T: type) void {
    std.debug.warn("in function > 0: {}\n", .{ @sizeOf(T) > 0 });
    std.debug.warn("in function > 0.01: {}\n", .{ @sizeOf(T) > 0.01 });
}

immediate > 0: false
immediate > 0.01: false
in function > 0: true
in function > 0.01: false

[andrewrk]

The fix is reverted in dad62a7 because it caused #4560. A new test case to cover the regression is added in e75598a.