Add stress vector access tests and correct problems discovered by it

  Major problem was not actually returning the proper register value
when running out of registers in a complex expression.

  Major change was to simplify otag_t size field for vectors, to no
longer store it in bytes, but in units size. The previous logic was
when it was still not changed to only support "jagged" vectors.

  Another problem was the proper correction of vector length calculation
during static initialization and/or incomplete initialization.