This was being incorrectly sized by the number of functions, not the highest label in use.

While investigating this, I've realized that the sanity checks to ensure there are no dead functions was never going to fire as the function list was built from the list of called functions. Instead I've changed the function scanning pass to look for labels starting a LABEL-RET pair.

Bug: b/125183107
Change-Id: Ic921097ed42a96b52f1ab7c9590c02fb3552b565
Reviewed-on: https://swiftshader-review.googlesource.com/c/25168Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>