Remove vconcat and vsplit clif instructions (#5465)

Fixes #5463.

* remove vsplit instruction

* remove vconcat instruction

* remove unsused half/double vector helper functions

* remove unused operand constraints

* delete + inline Type::half_vector method

cranelift/filetests/filetests/runtests/simd-vsplit.clif

@@ -1,31 +0,0 @@
-test interpret
-
-function %vsplit_i32x4_hi(i32x4) -> i32x2 {
-block0(v0: i32x4):
-    v1, v2 = vsplit.i32x4 v0
-    return v1
-}
-; run: %vsplit_i32x4_hi([1 2 3 4]) == [1 2]
-
-function %vsplit_i32x4_lo(i32x4) -> i32x2 {
-block0(v0: i32x4):
-    v1, v2 = vsplit.i32x4 v0
-    return v2
-}
-; run: %vsplit_i32x4_lo([1 2 3 4]) == [3 4]
-
-
-
-function %vsplit_scalar_i64x2_hi(i64x2) -> i64 {
-block0(v0: i64x2):
-    v1, v2 = vsplit.i64x2 v0
-    return v1
-}
-; run: %vsplit_scalar_i64x2_hi([1 2]) == 1
-
-function %vsplit_scalar_i64x2_lo(i64x2) -> i64 {
-block0(v0: i64x2):
-    v1, v2 = vsplit.i64x2 v0
-    return v2
-}
-; run: %vsplit_scalar_i64x2_lo([3 4]) == 4