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/reader/src/parser.rs

@@ -171,8 +171,7 @@ impl<'a> Context<'a> {
                     InstructionData::Nullary { .. } |
                     InstructionData::UnaryImm { .. } |
                     InstructionData::UnaryIeee32 { .. } |
-                    InstructionData::UnaryIeee64 { .. } |
-                    InstructionData::UnaryImmVector { .. } => {}
+                    InstructionData::UnaryIeee64 { .. } => {}
 
                     InstructionData::Unary { ref mut arg, .. } |
                     InstructionData::UnarySplit { ref mut arg, .. } |
@@ -1335,9 +1334,6 @@ impl<'a> Parser<'a> {
                     imm: self.match_ieee64("expected immediate 64-bit float operand")?,
                 }
             }
-            InstructionFormat::UnaryImmVector => {
-                unimplemented!();
-            }
             InstructionFormat::UnarySplit => {
                 InstructionData::UnarySplit {
                     opcode: opcode,