Various extensions to the vectorizer

Includes additions to the AstVectorizer:

• Vectorize assignments to vector symbols
• Vectorize loops nested within the vectorization axis if all bounds are independent of the axis
• Vectorize array subscripts if all indices and the array expression are independent of the axis
• Handle empty nodes (e.g. comments)

Additions to the x86 instruction set:

• Intrinsics for basic math functions (log, abs, etc.)
• Type conversions

And to the SelectIntrinsics pass:

• Add option to implemenet type conversions using __builtin_convertvector which is supported by most compilers instead of inserting intrinsics. Useful because not all conversions are supported natively on all CPUs.

tests/kernelcreation/test_functions.py

@@ -137,7 +137,7 @@ def test_binary_functions(gen_config, xp, function_name, dtype):
 
 dtype_and_target_for_integer_funcs = pytest.mark.parametrize(
     "dtype, target",
-    list(product([np.int32], AVAIL_TARGETS))
+    list(product([np.int32], [t for t in AVAIL_TARGETS if t is not Target.CUDA]))
     + list(
         product(
             [np.int64],