Add vsplit and vconcat instructions.

Add support for two new type variable functions: half_vector() and
double_vector().

Use these two instructions to break down unsupported SIMD types and
build them up again.

cranelift/docs/langref.rst

@@ -668,14 +668,9 @@ allocation pass and beyond.
 Vector operations
 -----------------
 
+.. autoinst:: vsplit
+.. autoinst:: vconcat
 .. autoinst:: vselect
-
-.. inst:: a = vbuild x, y, z, ...
-
-    Vector build.
-
-    Build a vector value from the provided lanes.
-
 .. autoinst:: splat
 .. autoinst:: insertlane
 .. autoinst:: extractlane