Allow for unencoded instructions in the binemit tests.

If an instruction doesn't have an associated encoding, use the standard
TargetIsa hook to encode it.

The test still fails if an instruction can't be encoded. There is no
legalization step.

cranelift/docs/testing.rst

@@ -336,3 +336,11 @@ that instruction is compared to the directive::
         [R#200c,%x8]        v11 = isub v1, v2       ; bin: 40628433
         return
     }
+
+If any instructions are unencoded (indicated with a `[-]` encoding field), they
+will be encoded using the same mechanism as the legalizer uses. However,
+illegal instructions for the ISA won't be expanded into other instruction
+sequences. Instead the test will fail.
+
+Value locations must be present if they are required to compute the binary
+bits. Missing value locations will cause the test to crash.