Add EntryRegDiversions to record diversions for each block entry.

cranelift/codegen/src/binemit/shrink.rs

@@ -20,7 +20,9 @@ pub fn shrink_instructions(func: &mut Function, isa: &dyn TargetIsa) {
     let mut divert = RegDiversions::new();
 
     for ebb in func.layout.ebbs() {
-        divert.clear();
+        // Load diversions from predecessors.
+        divert.at_ebb(&func.entry_diversions, ebb);
+
         for inst in func.layout.ebb_insts(ebb) {
             let enc = func.encodings[inst];
             if enc.is_legal() {