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/isa/riscv/expand-i32.cton

@@ -7,7 +7,7 @@ isa riscv supports_m=1
 set is_64bit=1
 isa riscv supports_m=1
 
-; regex: V=vx?\d+
+; regex: V=v\d+
 
 function carry_out(i32, i32) -> i32, b1 {
 ebb0(v1: i32, v2: i32):