Flatten the Value reference representation.

All values are now references into the value table, so drop the
distinction between direct and table values. Direct values don't exist
any more.

Also remove the parser support for the 'vxNN' syntax. Only 'vNN' values
can be parsed now.

docs/cton_lexer.py

@@ -47,10 +47,9 @@ class CretonneLexer(RegexLexer):
             # Well known value types.
             (r'\b(b\d+|i\d+|f32|f64)(x\d+)?\b', Keyword.Type),
             # v<nn> = value
-            # vx<nn> = value
             # ss<nn> = stack slot
             # jt<nn> = jump table
-            (r'(vx?|ss|jt)\d+', Name.Variable),
+            (r'(v|ss|jt)\d+', Name.Variable),
             # ebb<nn> = extended basic block
             (r'(ebb)\d+', Name.Label),
             # Match instruction names in context.