Popcnt vectorization

Hi Simon,

Same as in the other pull request, added the missing masks

Best regards,
Diego
Hi Diego, thanks for your contribution. I'm just testing the code on a Mac equipped with a CPU (i7-4850HQ) which supports AVX2. Surprisingly the command `sysctl -n machdep.cpu.features` does not list AVX2 as feature, but just:
`FPU VME DE PSE TSC MSR PAE MCE CX8 APIC SEP MTRR PGE MCA CMOV PAT PSE36 CLFSH DS ACPI MMX FXSR SSE SSE2 SS HTT TM PBE SSE3 PCLMULQDQ DTES64 MON DSCPL VMX SMX EST TM2 SSSE3 FMA CX16 TPR PDCM SSE4.1 SSE4.2 x2APIC MOVBE POPCNT AES PCID XSAVE OSXSAVE SEGLIM64 TSCTMR AVX1.0 RDRAND F16C`.  However AVX2 is listed in the output of `sysctl -n machdep.cpu`:

`
13
2147483656
GenuineIntel
Intel(R) Core(TM) i7-4850HQ CPU @ 2.30GHz
6
70
4
0
1
3219913727 2147154943
12219
739248384 33
263777
0
FPU VME DE PSE TSC MSR PAE MCE CX8 APIC SEP MTRR PGE MCA CMOV PAT PSE36 CLFSH DS ACPI MMX FXSR SSE SSE2 SS HTT TM PBE SSE3 PCLMULQDQ DTES64 MON DSCPL VMX SMX EST TM2 SSSE3 FMA CX16 TPR PDCM SSE4.1 SSE4.2 x2APIC MOVBE POPCNT AES PCID XSAVE OSXSAVE SEGLIM64 TSCTMR AVX1.0 RDRAND F16C
SMEP ENFSTRG RDWRFSGS TSC_THREAD_OFFSET BMI1 HLE AVX2 BMI2 INVPCID RTM
SYSCALL XD 1GBPAGE EM64T LAHF RDTSCP TSCI
16
8
15
5
64
64
3
270624
1
1
1
2
1
1
1
1
0
1
7 832 832 0
3
4
48
7
0
3
48
64
8
256
8
64
64
1024
39
48
4
8
`

So maybe just a match on the latter output?