* Fix codegen for splat - instead of giving vectors of length N
   to shufflevector for both of the operands, it gives vectors of length
   1. The mask vector is the only one that needs N elements.
 * Separate Splat into SplatSrc and SplatGen; the `len` is not needed
   once it gets to codegen since it is redundant with the result type.
 * Refactor compile error for wrong vector element type so that the
   compile error message is not duplicated in zig source code
 * Improve implementation to correctly handle comptime values such as
   undefined and lazy values.
 * Improve compile error for bad vector element type to point to the
   correct place.
 * Delete dead code.
 * Modify behavior test to use an array cast instead of vector element
   indexing since I'm merging this splat commit out-of-order from
   Shawn's patch set.