Add length checks to volk_8u_x2_encodeframepolar_8u

[argilo]

Fixes #414.

The various volk_8u_x2_encodeframepolar_8u implementations have warnings about the input size:

volk/kernels/volk/volk_8u_x2_encodeframepolar_8u.h

Lines 157 to 158 in a26a1b8

	// This last part requires at least 16-bit frames.
	// Smaller frames are useless for SIMD optimization anyways. Just choose GENERIC!

volk/kernels/volk/volk_8u_x2_encodeframepolar_8u.h

Lines 430 to 431 in a26a1b8

	// This last part requires at least 32-bit frames.
	// Smaller frames are useless for SIMD optimization anyways. Just choose GENERIC!

volk/kernels/volk/volk_8u_x2_encodeframepolar_8u.h

Lines 692 to 693 in a26a1b8

	// This last part requires at least 16-bit frames.
	// Smaller frames are useless for SIMD optimization anyways. Just choose GENERIC!

volk/kernels/volk/volk_8u_x2_encodeframepolar_8u.h

Lines 964 to 965 in a26a1b8

	// This last part requires at least 32-bit frames.
	// Smaller frames are useless for SIMD optimization anyways. Just choose GENERIC!

So let's heed those warnings by falling back to the generic implementation if the input size is too small.

Doing so also removes the need for extra size checks in volk_32f_8u_polarbutterfly_32f.h.

After this change, all failures disappear. I tested with the following:

for i in {1..64}; do apps/volk_profile -n -i 1 -v $i -R encodepolar | grep fail; done

I also verified that all valgrind warnings disappear after these changes. I tested with:

for i in {1..64}; do valgrind apps/volk_profile -n -i 1 -v $i -R encodepolar | grep fail; done

[argilo]

May as well call the AVX2 version here, since we're in volk_32f_8u_polarbutterfly_32f_u_avx2.

[argilo]

I also confirmed that GNU Radio's qa_polar_decoder_sc_systematic_test test passes after this change, even when VOLK_GENERIC=1 is removed:

diff --git a/cmake/Modules/GrTest.cmake b/cmake/Modules/GrTest.cmake
index b5b2c196ec..f31e199067 100644
--- a/cmake/Modules/GrTest.cmake
+++ b/cmake/Modules/GrTest.cmake
@@ -67,7 +67,7 @@ function(GR_ADD_TEST test_name)
     # We add it to the beginning of the list to use locally-built modules before installed ones.
     list(INSERT pypath 0 "${PROJECT_BINARY_DIR}/test_modules")
 
-    set(environs "VOLK_GENERIC=1" "GR_DONT_LOAD_PREFS=1" "srcdir=${srcdir}"
+    set(environs "GR_DONT_LOAD_PREFS=1" "srcdir=${srcdir}"
                  "GR_CONF_CONTROLPORT_ON=False")
     list(APPEND environs ${GR_TEST_ENVIRONS})
 

[jdemel]

LGTM. When I implemented these kernels, these very small sizes seemed to be useless for an FEC code. But people stumble over it anyways. Thus, we should fix that.

[argilo]

If GNU Radio's qa_polar_decoder_sc_systematic_test test uses an unrealistic code size, maybe it should be fixed.

[argilo]

Another motivation for making this change is that it would allow a CI test to sweep over small vector lengths (which are prone to bugs) in all kernels.