x86/itx: Add 12-bit 4x4 transforms in AVX2

Refactors itx into separate 10, 12 bit functions to prevent conditional
jumps.

inv_txfm_add_4x4_adst_adst_0_12bpc_c: 370.9
inv_txfm_add_4x4_adst_adst_0_12bpc_avx2: 68.6
inv_txfm_add_4x4_adst_adst_1_12bpc_c: 371.0
inv_txfm_add_4x4_adst_adst_1_12bpc_avx2: 68.7
inv_txfm_add_4x4_adst_dct_0_12bpc_c: 413.1
inv_txfm_add_4x4_adst_dct_0_12bpc_avx2: 69.2
inv_txfm_add_4x4_adst_dct_1_12bpc_c: 412.7
inv_txfm_add_4x4_adst_dct_1_12bpc_avx2: 68.8
inv_txfm_add_4x4_adst_flipadst_0_12bpc_c: 378.5
inv_txfm_add_4x4_adst_flipadst_0_12bpc_avx2: 74.9
inv_txfm_add_4x4_adst_flipadst_1_12bpc_c: 378.1
inv_txfm_add_4x4_adst_flipadst_1_12bpc_avx2: 74.6
inv_txfm_add_4x4_adst_identity_0_12bpc_c: 347.8
inv_txfm_add_4x4_adst_identity_0_12bpc_avx2: 48.8
inv_txfm_add_4x4_adst_identity_1_12bpc_c: 342.7
inv_txfm_add_4x4_adst_identity_1_12bpc_avx2: 49.0
inv_txfm_add_4x4_dct_adst_0_12bpc_c: 399.2
inv_txfm_add_4x4_dct_adst_0_12bpc_avx2: 73.1
inv_txfm_add_4x4_dct_adst_1_12bpc_c: 398.7
inv_txfm_add_4x4_dct_adst_1_12bpc_avx2: 72.2
inv_txfm_add_4x4_dct_dct_0_12bpc_c: 69.6
inv_txfm_add_4x4_dct_dct_0_12bpc_avx2: 32.9
inv_txfm_add_4x4_dct_dct_1_12bpc_c: 420.5
inv_txfm_add_4x4_dct_dct_1_12bpc_avx2: 72.2
inv_txfm_add_4x4_dct_flipadst_0_12bpc_c: 405.5
inv_txfm_add_4x4_dct_flipadst_0_12bpc_avx2: 75.9
inv_txfm_add_4x4_dct_flipadst_1_12bpc_c: 404.2
inv_txfm_add_4x4_dct_flipadst_1_12bpc_avx2: 75.6
inv_txfm_add_4x4_dct_identity_0_12bpc_c: 374.1
inv_txfm_add_4x4_dct_identity_0_12bpc_avx2: 51.6
inv_txfm_add_4x4_dct_identity_1_12bpc_c: 368.0
inv_txfm_add_4x4_dct_identity_1_12bpc_avx2: 51.8
inv_txfm_add_4x4_flipadst_adst_0_12bpc_c: 368.0
inv_txfm_add_4x4_flipadst_adst_0_12bpc_avx2: 69.2
inv_txfm_add_4x4_flipadst_adst_1_12bpc_c: 370.7
inv_txfm_add_4x4_flipadst_adst_1_12bpc_avx2: 70.4
inv_txfm_add_4x4_flipadst_dct_0_12bpc_c: 393.7
inv_txfm_add_4x4_flipadst_dct_0_12bpc_avx2: 70.1
inv_txfm_add_4x4_flipadst_dct_1_12bpc_c: 392.9
inv_txfm_add_4x4_flipadst_dct_1_12bpc_avx2: 69.6
inv_txfm_add_4x4_flipadst_flipadst_0_12bpc_c: 382.2
inv_txfm_add_4x4_flipadst_flipadst_0_12bpc_avx2: 74.6
inv_txfm_add_4x4_flipadst_flipadst_1_12bpc_c: 381.3
inv_txfm_add_4x4_flipadst_flipadst_1_12bpc_avx2: 74.9
inv_txfm_add_4x4_flipadst_identity_0_12bpc_c: 346.7
inv_txfm_add_4x4_flipadst_identity_0_12bpc_avx2: 48.2
inv_txfm_add_4x4_flipadst_identity_1_12bpc_c: 347.9
inv_txfm_add_4x4_flipadst_identity_1_12bpc_avx2: 48.7
inv_txfm_add_4x4_identity_adst_0_12bpc_c: 344.7
inv_txfm_add_4x4_identity_adst_0_12bpc_avx2: 59.8
inv_txfm_add_4x4_identity_adst_1_12bpc_c: 340.5
inv_txfm_add_4x4_identity_adst_1_12bpc_avx2: 59.2
inv_txfm_add_4x4_identity_dct_0_12bpc_c: 369.8
inv_txfm_add_4x4_identity_dct_0_12bpc_avx2: 59.3
inv_txfm_add_4x4_identity_dct_1_12bpc_c: 369.5
inv_txfm_add_4x4_identity_dct_1_12bpc_avx2: 59.2
inv_txfm_add_4x4_identity_flipadst_0_12bpc_c: 353.4
inv_txfm_add_4x4_identity_flipadst_0_12bpc_avx2: 65.6
inv_txfm_add_4x4_identity_flipadst_1_12bpc_c: 350.9
inv_txfm_add_4x4_identity_flipadst_1_12bpc_avx2: 65.9
inv_txfm_add_4x4_identity_identity_0_12bpc_c: 326.1
inv_txfm_add_4x4_identity_identity_0_12bpc_avx2: 39.5
inv_txfm_add_4x4_identity_identity_1_12bpc_c: 321.6
inv_txfm_add_4x4_identity_identity_1_12bpc_avx2: 39.5