x86-64: optimize certain commutative VEX-encoded insns

x86-64: optimize certain commutative VEX-encoded insns

When they're in the 0F opcode space, swapping their source operands may
allow switching from 3-byte to 2-byte VEX prefix encoding. Note that NaN
behavior precludes us doing so for many packed and scalar floating point
insns; such an optimization would need to be done by the compiler
instead in this case, when it knows that NaN-s have undefined behavior
anyway.

While for explicitly specified AVX/AVX2 insns the optimization (for now
at least) gets done only for -O2 and -Os, it is utilized by default in
SSE2AVX mode, as there we're re-writing the programmer's specified insns
anyway.

Rather than introducing a new attribute flag, the change re-uses one
which so far was meaningful only for EVEX-encoded insns.