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.

filetests/parser/rewrite.cton

@@ -12,13 +12,13 @@ test cat
 function defs() {
 ebb100(v20: i32):
     v1000 = iconst.i32x8 5
-    vx200 = f64const 0x4.0p0
+    v9200 = f64const 0x4.0p0
     trap
 }
 ; sameln: function defs() {
 ; nextln: $ebb100($v20: i32):
 ; nextln:     $v1000 = iconst.i32x8 5
-; nextln:     $vx200 = f64const 0x1.0000000000000p2
+; nextln:     $v9200 = f64const 0x1.0000000000000p2
 ; nextln:     trap
 ; nextln: }