Legalize unsigned-to-float conversions for Intel 64.

Also make sure we generate type checks for the controlling type variable
in legalization patterns. This is not needed for encodings since the
encoding tables are already keyed on the controlling type variable.

lib/cretonne/meta/gen_legalizer.py

@@ -199,7 +199,7 @@ def unwrap_inst(iref, node, fmt):
                     n = expr.inst.value_opnums.index(opnum)
                     fmt.format('dfg.resolve_aliases(args[{}]),', n)
             # Evaluate the instruction predicate, if any.
-            instp = expr.inst_predicate()
+            instp = expr.inst_predicate_with_ctrl_typevar()
             fmt.line(instp.rust_predicate(0) if instp else 'true')
         fmt.outdented_line('} else {')
         fmt.line('unreachable!("bad instruction format")')