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/builder.rs

@@ -6,7 +6,7 @@
 use ir::{types, instructions};
 use ir::{InstructionData, DataFlowGraph, Cursor};
 use ir::{Opcode, Type, Inst, Value, Ebb, JumpTable, SigRef, FuncRef, ValueList};
-use ir::immediates::{Imm64, Uimm8, Ieee32, Ieee64, ImmVector};
+use ir::immediates::{Imm64, Uimm8, Ieee32, Ieee64};
 use ir::condcodes::{IntCC, FloatCC};
 
 /// Base trait for instruction builders.