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/flowgraph.rs

@@ -120,7 +120,7 @@ impl ControlFlowGraph {
     }
 
     fn compute_block(&mut self, func: &Function, block: Block) {
-        for inst in func.layout.block_insts(block) {
+        for inst in func.layout.block_likely_branches(block) {
             match func.dfg.analyze_branch(inst) {
                 BranchInfo::SingleDest(dest, _) => {
                     self.add_edge(block, inst, dest);