JIT: Optimize generated functions earlier to lower memory usage

Hello,

This is to introduce a patch to lower the memory footprint of JITed
code by optimizing functions at the function level (i.e. with
function-level optimization passes) as soon as they are generated.
This addresses the code comment inside llvm_optimize_module():

/*
 * Do function level optimization. This could be moved to the point where
 * functions are emitted, to reduce memory usage a bit.
 */
 LLVMInitializeFunctionPassManager(llvm_fpm);