If we have a vector of all zeros, a __setzero_* function call is emitted,
permitting calling specialized intrinsics for this. Undefined values
are reflected with an __undef_* call, which similarly allows passing that
information along.
This change also includes a cleanup to the signature of the __smear_*
functions; since they already have different names depending on the
scalar value type, we don't need to use the trick of passing an
undefined value of the return vector type as the first parameter as
an indirect way to overload by return value.
Issue #317.
Fixes to __load and __store.
Added __add, __mul, __equal, __not_equal, __extract_elements, __smear_i64, __cast_sext, __cast_zext,
and __scatter_base_offsets32_float.
__rcp_varying_float now has a fast-math and full-precision implementation.
