[BOLT] Fix assertion when setting size of jump table symbol

Summary: This assertion was making sure that when we patched up symbol sizes that we wouldn't modify the size of a symbol that has already had its size set.  The issue here is that private symbols are sometimes composed of multiple objects internally (e.g. jump tables).  In this particular case a jump table started at the same address as the private data blob it was contained in.  Currently, there isn't any good way of differentiating symbols that start at the same address (except possibly using multimaps for certain data structures).  I'm hacking around it by modifying the assertion to ignore jump tables and skip setting the size when it has already been set.  This shouldn't affect any existing optimizations since the only thing that depended on sizes is data reordering and that currently ignores jump tables and private data blobs.

(cherry picked from FBD7269207)