i#1312 AVX-512 support: Fix VEX.L0/VEX.L1 in mask opcode encodings.

Fixes a bug where we didn't respect and check encodings rules for the VEX.L bit.

The VEX encoded AVX-512 instructions had not been ground-truth tested. The opcodes have now been checked against llvm-mc and binutils/gas/objdump.

Issue: #1312