Skip non-branching blocks now that we're using basic blocks

This is a rebase of [1]. In the long term, we'll want to simplify these
analysis passes. For now, this is simple and will reduce the number of
instructions processed in certain cases.

[1] https://github.com/bytecodealliance/cranelift/pull/866

cranelift/codegen/src/cfg_printer.rs

@@ -53,7 +53,7 @@ impl<'a> CFGPrinter<'a> {
             write!(w, "    {} [shape=record, label=\"{{", block)?;
             crate::write::write_block_header(w, self.func, None, block, 4)?;
             // Add all outgoing branch instructions to the label.
-            for inst in self.func.layout.block_insts(block) {
+            for inst in self.func.layout.block_likely_branches(block) {
                 write!(w, " | <{}>", inst)?;
                 PlainWriter.write_instruction(w, self.func, &aliases, None, inst, 0)?;
             }