memcmp: remove unreachable code from memcmp simd

memcmp: remove unreachable code from memcmp simd

cppcheck:

src/util-memcmp.h:281:18: warning: Identical condition 'len-offset<16', second condition is always false [identicalConditionAfterEarlyExit]
        if (diff < 16) {
                 ^
src/util-memcmp.h:280:24: note: 'diff' is assigned value 'len-offset' here.
        int diff = len - offset;
                       ^
src/util-memcmp.h:269:33: note: If condition 'len-offset<16' is true, the function will return/exit
        if (likely(len - offset < 16)) {
                                ^
src/util-memcmp.h:281:18: note: Testing identical condition 'len-offset<16'
        if (diff < 16) {
                 ^
src/util-memcmp.h:344:18: warning: Identical condition 'len-offset<16', second condition is always false [identicalConditionAfterEarlyExit]
        if (diff < 16) {
                 ^
src/util-memcmp.h:343:24: note: 'diff' is assigned value 'len-offset' here.
        int diff = len - offset;
                       ^
src/util-memcmp.h:318:33: note: If condition 'len-offset<16' is true, the function will return/exit
        if (likely(len - offset < 16)) {
                                ^
src/util-memcmp.h:344:18: note: Testing identical condition 'len-offset<16'
        if (diff < 16) {
                 ^
src/util-memcmp.h:171:18: warning: Identical condition 'len-offset<16', second condition is always false [identicalConditionAfterEarlyExit]
        if (diff < 16) {
                 ^
src/util-memcmp.h:170:24: note: 'diff' is assigned value 'len-offset' here.
        int diff = len - offset;
                       ^
src/util-memcmp.h:159:33: note: If condition 'len-offset<16' is true, the function will return/exit
        if (likely(len - offset < 16)) {
                                ^
src/util-memcmp.h:171:18: note: Testing identical condition 'len-offset<16'
        if (diff < 16) {
                 ^
src/util-memcmp.h:233:18: warning: Identical condition 'len-offset<16', second condition is always false [identicalConditionAfterEarlyExit]
        if (diff < 16) {
                 ^
src/util-memcmp.h:232:24: note: 'diff' is assigned value 'len-offset' here.
        int diff = len - offset;
                       ^
src/util-memcmp.h:208:33: note: If condition 'len-offset<16' is true, the function will return/exit
        if (likely(len - offset < 16)) {
                                ^
src/util-memcmp.h:233:18: note: Testing identical condition 'len-offset<16'
        if (diff < 16) {
                 ^

(cherry picked from commit ca97ed44361fe84cae72fad1807825f149f983eb)