arm64: mc: NEON implementation of w_mask for 16 bpc

Checkasm numbers:          Cortex A53       A72       A73
w_mask_420_w4_16bpc_neon:       173.6     123.5     120.3
w_mask_420_w8_16bpc_neon:       484.2     344.1     329.5
w_mask_420_w16_16bpc_neon:     1411.2    1027.4    1035.1
w_mask_420_w32_16bpc_neon:     5561.5    4093.2    3980.1
w_mask_420_w64_16bpc_neon:    13809.6    9856.5    9581.0
w_mask_420_w128_16bpc_neon:   35614.7   25553.8   24284.4
w_mask_422_w4_16bpc_neon:       159.4     112.2     114.2
w_mask_422_w8_16bpc_neon:       453.4     326.1     326.7
w_mask_422_w16_16bpc_neon:     1394.6    1062.3    1050.2
w_mask_422_w32_16bpc_neon:     5485.8    4219.6    4027.3
w_mask_422_w64_16bpc_neon:    13701.2   10079.6    9692.6
w_mask_422_w128_16bpc_neon:   35455.3   25892.5   24625.9
w_mask_444_w4_16bpc_neon:       153.0     112.3     112.7
w_mask_444_w8_16bpc_neon:       437.2     331.8     325.8
w_mask_444_w16_16bpc_neon:     1395.1    1069.1    1041.7
w_mask_444_w32_16bpc_neon:     5370.1    4213.5    4138.1
w_mask_444_w64_16bpc_neon:    13482.6   10190.5   10004.6
w_mask_444_w128_16bpc_neon:   35583.7   26911.2   25638.8

Corresponding numbers for 8 bpc for comparison:

w_mask_420_w4_8bpc_neon:        126.6      79.1      87.7
w_mask_420_w8_8bpc_neon:        343.9     195.0     211.5
w_mask_420_w16_8bpc_neon:       886.3     540.3     577.7
w_mask_420_w32_8bpc_neon:      3558.6    2152.4    2216.7
w_mask_420_w64_8bpc_neon:      8894.9    5161.2    5297.0
w_mask_420_w128_8bpc_neon:    22520.1   13514.5   13887.2
w_mask_422_w4_8bpc_neon:        112.9      68.2      77.0
w_mask_422_w8_8bpc_neon:        314.4     175.5     208.7
w_mask_422_w16_8bpc_neon:       835.5     565.0     608.3
w_mask_422_w32_8bpc_neon:      3381.3    2231.8    2287.6
w_mask_422_w64_8bpc_neon:      8499.4    5343.6    5460.8
w_mask_422_w128_8bpc_neon:    21823.3   14206.5   14249.1
w_mask_444_w4_8bpc_neon:        104.6      65.8      72.7
w_mask_444_w8_8bpc_neon:        290.4     173.7     196.6
w_mask_444_w16_8bpc_neon:       831.4     586.7     591.7
w_mask_444_w32_8bpc_neon:      3320.8    2300.6    2251.0
w_mask_444_w64_8bpc_neon:      8300.0    5480.5    5346.8
w_mask_444_w128_8bpc_neon:    21633.8   15981.3   14384.8