[X86] Simplify the TSFlags checking code in EvexToVexInstPass. NFCI

The code was previously checking the L2 and L flag on 3 separate lines, treating the combination as an encoding. Instead its better to think of the L2 bit as being something that can't be done with VEX and early returning. Then we just need to check the L bit.

llvm-svn: 335015