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_type_cast.py

@@ -61,8 +61,13 @@ def test_type_cast(gen_config, xp, from_type, to_type):
     kfunc = kernel.compile()
     kfunc(inp=inp, outp=outp)
 
-    # rounding mode depends on platform
-    try:
+    if np.issubdtype(from_type, np.floating) and not np.issubdtype(
+        to_type, np.floating
+    ):
+        # rounding mode depends on platform
+        try:
+            xp.testing.assert_array_equal(outp, truncated)
+        except AssertionError:
+            xp.testing.assert_array_equal(outp, rounded)
+    else:
         xp.testing.assert_array_equal(outp, truncated)
-    except AssertionError:
-        xp.testing.assert_array_equal(outp, rounded)