* `@as` and `@bitCast` no longer unconditionally return `true` from
   this function; they forward the question to their sub-expression.
 * fix `@splat` incorrectly being marked as needing a memory location
   (this function returns a SIMD vector; it definitely does not want a
   memory location).

Makes AstGen generate slightly nicer ZIR, which in turn generates
slightly nicer AIR, generating slightly nicer machine code in debug
builds.

It also means I can procrastinate implementing the bitcast_result_ptr
ZIR instruction semantic analysis :^)