x86: drop use of VEX3SOURCES

The attribute really specifies that the sum of register and memory
operands is 4. Express it like that in most places, while using the 2nd
(apart from XOP) CPU feature flags (FMA4) in reversed operand matching
logic.

With the use in build_modrm_byte() gone, part of an assertion there
also becomes meaningless - simplify that at the same time.

With all uses of the opcode modifier field gone, also drop that.