Always create live ranges for dead EBB arguments.

The live value tracker expects them to be there.

We may eventually delete dead arguments from internal EBBs, but at least
the entry block needs to be able to handle dead function arguments.

cranelift/filetests/regalloc/basic.cton

@@ -10,3 +10,10 @@ ebb0(v1: i32, v2: i32):
 ; sameln: iadd
     return
 }
+
+; Function with a dead argument.
+function dead_arg(i32, i32) -> i32{
+ebb0(v1: i32, v2: i32):
+; check: return $v1
+    return v1
+}