Remove the vconst instruction and the UnaryImmVector format.

No instruction sets actually have single instructions for materializing
vector constants. You always need to use a constant pool.

Cretonne doesn't have constant pools yet, but it will in the future, and
that is how vector constants should be represented.

lib/cretonne/src/write.rs

@@ -233,7 +233,6 @@ fn write_instruction(w: &mut Write,
         UnaryImm { imm, .. } => writeln!(w, " {}", imm),
         UnaryIeee32 { imm, .. } => writeln!(w, " {}", imm),
         UnaryIeee64 { imm, .. } => writeln!(w, " {}", imm),
-        UnaryImmVector { ref data, .. } => writeln!(w, " {}", data),
         UnarySplit { arg, .. } => writeln!(w, " {}", arg),
         Binary { args, .. } => writeln!(w, " {}, {}", args[0], args[1]),
         BinaryImm { arg, imm, .. } => writeln!(w, " {}, {}", arg, imm),