* 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.
005a54a853