Update ref-op pruning pass to use remarks API for stats tracking

[stuartarchibald]

Following on from #837, which adds support for obtaining remarks from LLVM passes. It should now be possible to update the RefPrunePass, which does Numba specific ref-op pruning, to emit remarks and for them to be consumed by this API. This would effectively replace this:

llvmlite/ffi/custom_passes.cpp

Lines 163 to 166 in dfca79f

	static size_t stats_per_bb;
	static size_t stats_diamond;
	static size_t stats_fanout;
	static size_t stats_fanout_raise;

and this:

llvmlite/ffi/custom_passes.cpp

Lines 1178 to 1210 in dfca79f

	/**
	* Struct for holding statistics about the amount of pruning performed by
	* each type of pruning algorithm.
	*/
	typedef struct PruneStats {
	size_t basicblock;
	size_t diamond;
	size_t fanout;
	size_t fanout_raise;
	} PRUNESTATS;
	API_EXPORT(void)
	LLVMPY_DumpRefPruneStats(PRUNESTATS *buf, bool do_print) {
	/* PRUNESTATS is updated with the statistics about what has been pruned from
	* the RefPrunePass static state vars. This isn't threadsafe but neither is
	* the LLVM pass infrastructure so it's all done under a python thread lock.
	*
	* do_print if set will print the stats to stderr.
	*/
	if (do_print) {
	errs() << "refprune stats "
	<< "per-BB " << RefPrunePass::stats_per_bb << " "
	<< "diamond " << RefPrunePass::stats_diamond << " "
	<< "fanout " << RefPrunePass::stats_fanout << " "
	<< "fanout+raise " << RefPrunePass::stats_fanout_raise << " "
	<< "\n";
	};
	buf->basicblock = RefPrunePass::stats_per_bb;
	buf->diamond = RefPrunePass::stats_diamond;
	buf->fanout = RefPrunePass::stats_fanout;
	buf->fanout_raise = RefPrunePass::stats_fanout_raise;
	}

and also allowing tracking of the source line origin of the pruned ref-ops (which is interesting in itself).