Update `sse2_copy_upper` to use a single bitmask

pyfastani/_sequtils/sse2.c

@@ -2,17 +2,11 @@
 #include <x86intrin.h>
 
 extern void sse2_copy_upper(char* dst, const char* src, size_t len) {
-    const __m128i ascii_a = _mm_set1_epi8('a' - 1);
-    const __m128i ascii_z = _mm_set1_epi8('z');
     const __m128i offset  = _mm_set1_epi8('a' - 'A');
 
     while (len >= sizeof(__m128i)) {
         __m128i inp = _mm_loadu_si128((__m128i*) src);
-        __m128i greater_than_a = _mm_cmpgt_epi8(inp, ascii_a);
-        __m128i less_equal_z = _mm_cmpgt_epi8(ascii_z, inp);
-        __m128i mask = _mm_and_si128(greater_than_a, less_equal_z);
-        __m128i diff = _mm_and_si128(mask, offset);
-        __m128i added = _mm_sub_epi8(inp, diff);
+        __m128i added = _mm_andnot_si128(offset, inp);
         _mm_storeu_si128((__m128i *) dst, added);
         len -= sizeof(__m128i);
         src += sizeof(__m128i);