CL 1282523002 changed the bitcode parser from using a vector, to using
an unordered map. This was done because one could forward reference a
local variable, and would freeze the computer trying to allocate a
vector large enough to contain the index.

This patch goes back to using vectors. To fix the forward variable
reference, we use the number of bytes in the function to determine if
the index is possible. This stops very large (probematic) vector
resizes from happening.

BUG=None
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1293923002 .