Detach mapping to local in ctx_set_local_type

Similar to the previous fix to ctx_clear_local_types, we must detach
mappings to a local if we are changing its value.

bootstraptest/test_yjit.rb

@@ -1599,3 +1599,16 @@ assert_equal '10', %q{
 
   val
 }
+
+# regression test of local type change
+assert_equal '1.1', %q{
+def bar(baz, quux)
+  if baz.integer?
+    baz, quux = quux, nil
+  end
+  baz.to_s
+end
+
+bar(123, 1.1)
+bar(123, 1.1)
+}

yjit_core.c

@@ -266,6 +266,15 @@ void ctx_set_local_type(ctx_t* ctx, size_t idx, val_type_t type)
     if (idx >= MAX_LOCAL_TYPES)
         return;
 
+    // If any values on the stack map to this local we must detach them
+    for (int i = 0; i < MAX_TEMP_TYPES; i++) {
+        temp_mapping_t *mapping = &ctx->temp_mapping[i];
+        if (mapping->kind == TEMP_LOCAL && mapping->idx == idx) {
+            ctx->temp_types[i] = ctx->local_types[mapping->idx];
+            *mapping = MAP_STACK;
+        }
+    }
+
     ctx->local_types[idx] = type;
 }