Do not bother to emit a BytecodeBlock for an empty symbol table. This commonly

Do not bother to emit a BytecodeBlock for an empty symbol table.  This commonly
occurs when the symbol table for a module has been stripped, making all of the
function local symbols go away.

This saves 6728 bytes in the stripped bytecode file of 254.gap (which obviously
has 841 functions), which isn't a ton, but helps and was easy.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10750 91177308-0d34-0410-b5e6-96231b3b80d8