[X86] Teach assembler parser to accept lsl and lar with a 64 or 32 source...

[X86] Teach assembler parser to accept lsl and lar with a 64 or 32 source register when the destination is a 64 register.

Previously we only accepted a 32-bit source with a 64-bit dest.

Accepting 64-bit as well is more consistent with gas behavior. I
think maybe we should accept 16 bit register as well, but I'm not
sure.