meshletcodec: Remove extra null checks for clang via assume

In decodeMeshletSimd, data can not be null (even if input buffer is
NULL/0, this is invalid as minimum encoded size is 16 bytes).

decodeVerticesSimd and decodeTrianglesSimd may return NULL if they
encounter a bounds error; however, if they exit normally, we do not need
to do if (!data) check. This is not obvious to the compiler from the
regular function flow here unless data is explicitly marked as non-null;
builtin_assume does that for clang which streamlines the branch flow
here.

Author: zeux

src/meshletcodec.cpp

@@ -872,6 +872,11 @@ SIMD_TARGET static int decodeMeshletSimd(void* vertices, void* triangles, const
 	assert(gDecodeTablesInitialized);
 	(void)gDecodeTablesInitialized;
 
+#ifdef __clang__
+	// data is guaranteed to be non-null initially; if decode loops never hit bounds errors, it remains non-null
+	__builtin_assume(data);
+#endif
+
 	// decodes 4 vertices at a time with tail processing; writes up to align(vertex_size * vertex_count, 4)
 	// raw decoding skips tail processing by rounding up vertex count; it's safe because output buffer is guaranteed to have extra space, and tail control data is 0
 	if (vertex_size == 4 || Raw)