This fixes PR531, a crash when running the CBE on a bytecode file.

The problem is that Function::renameLocalSymbols is iterating through
the symbol table planes, occasionally calling setName to rename a value
(which used to do a symbol table remove/insert pair).

The problem is that if there is only a single value in a particular type
plane that the remove will nuke the symbol table plane, and the insert
will create and insert a new one.  This hoses Function::renameLocalSymbols
because it has an iterator to the old plane, under the (very reasonable)
assumption that simply renaming a value won't cause the type plane to
disappear.

This patch fixes the bug by making the rename operation a single atomic
operation, which has a side effect of making the whole thing faster too. :)

llvm-svn: 20469