x86-64: restrict by-imm31 optimization

Avoid changing the encoding when there's no size gain: If there's a REX
or REX2 prefix anyway and the base opcode wouldn't be changed, dropping
just REX.W / REX2.W has no (size) effect. (Same for the AND-by-imm7 case
in the same big conditional.)

While there also pull out the .qword check: For the 2-register-operands
case whether that's done on the 1st or 2nd operand doesn't matter. Due
to reduction in necessary parentheses this improves readability a tiny
bit.