fixups for @splat

* 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.

test/compile_errors.zig

@@ -6514,7 +6514,7 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
         \\    var v = @splat(4, c);
         \\}
     ,
-        "tmp.zig:3:20: error: vector element type must be integer, float, bool, or pointer; 'comptime_int' is invalid",
+        "tmp.zig:3:23: error: vector element type must be integer, float, bool, or pointer; 'comptime_int' is invalid",
     );
 
     cases.add("compileLog of tagged enum doesn't crash the compiler",