Disallow branching to the entry block.

Functions that would otherwise start with a loop should start with a
separate ebb which just branches to the header of the loop.

cranelift/filetests/licm/basic.cton

@@ -2,30 +2,33 @@ test licm
 
 function %simple_loop(i32) -> i32 {
 
-ebb1(v0: i32):
-    v1 = iconst.i32 1
-    v2 = iconst.i32 2
-    v3 = iadd v1, v2
-    brz v0, ebb2(v0)
-    v4 = isub v0, v1
-    jump ebb1(v4)
+ebb0(v0: i32):
+    jump ebb1(v0)
 
-ebb2(v5: i32):
-    return v5
+ebb1(v1: i32):
+    v2 = iconst.i32 1
+    v3 = iconst.i32 2
+    v4 = iadd v2, v3
+    brz v1, ebb2(v1)
+    v5 = isub v1, v2
+    jump ebb1(v5)
+
+ebb2(v6: i32):
+    return v6
 
 }
 ; sameln: function %simple_loop
-; nextln: ebb2(v6: i32):
-; nextln:     v1 = iconst.i32 1
-; nextln:     v2 = iconst.i32 2
-; nextln:     v3 = iadd v1, v2
-; nextln:     jump ebb0(v6)
-; nextln: 
 ; nextln: ebb0(v0: i32):
-; nextln:     brz v0, ebb1(v0)
-; nextln:     v4 = isub v0, v1
-; nextln:     jump ebb0(v4)
+; nextln:     v2 = iconst.i32 1
+; nextln:     v3 = iconst.i32 2
+; nextln:     v4 = iadd v2, v3
+; nextln:     jump ebb1(v0)
 ; nextln: 
-; nextln: ebb1(v5: i32):
-; nextln:     return v5
+; nextln: ebb1(v1: i32):
+; nextln:     brz v1, ebb2(v1)
+; nextln:     v5 = isub v1, v2
+; nextln:     jump ebb1(v5)
+; nextln: 
+; nextln: ebb2(v6: i32):
+; nextln:     return v6
 ; nextln: }