Fix bounds checking for 16-bit targets (Issue #2520)

[luismarques]

Fixes Issue #2520

This undoes the automatic zero extension of the index value used in the array bounds checking (back to size_t, which will not be a 32-bit for 16-bit targets). That fixes an internal LDC error that occurs when compiling, with bounds checking on, code like this:

void test()
{
    int[1] array;
    ushort k = 0;
    auto value = array[k];
}

[JohanEngelen]

Needs a testcase.

[luismarques]

Now with a test.

[kinke]

I think it makes sense to investigate where the zero-extension from i16 to i32 comes from, as this may just be a symptom of a more general issue. Studying the -vv output, starting from the back, should give insight.