Why does Matlab check the content of an if statement, that is always false?

I observed that Matlab obviously needs execution time for code that will never be used. Moreover, the performance depends on that code (that will never be used). I would like to know, why the different codes (see examples below) lead to such different execution times and how this effect can be avoided.

As a minimal example, consider the following 5 cases, which lead to the following runtimes on my machine:

• Case1 : 0.005680 seconds.
• Case2 : 0.029010 seconds.
• Case3 : 0.275790 seconds.
• Case4 : 3.849741 seconds.
• Case5 : 5.243183 seconds.

%--------Case 1---------
tic
for k=1:10^6
    if false
    end
    a=1;
end
toc

%--------Case 2---------
tic
for k=1:10^6
    if false
        svd(rand(100));
    end
end
toc

%--------Case 3---------
tic
for k=1:10^6
    if false
        svd(rand(100));
    end
    a=1;
end
toc

%--------Case 4---------
tic
for k=1:10^6
    if false
        svd(rand(100))      %without ";"
    end
end
toc

%--------Case 5---------
tic
for k=1:10^6
    if false
        svd(rand(100))
    end
    a=1;
end
toc