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/ir/immediates.rs

@@ -434,12 +434,6 @@ impl FromStr for Ieee64 {
     }
 }
 
-/// Arbitrary vector immediate.
-///
-/// This kind of immediate can represent any kind of SIMD vector constant.
-/// The representation is simply the sequence of bytes that would be used to store the vector.
-pub type ImmVector = Vec<u8>;
-
 #[cfg(test)]
 mod tests {
     use super::*;