x86: Drop SwapSources

gas/ChangeLog:

        * config/tc-i386.c (build_modrm_byte): Dropped the use of
	SWAP_SOURCES to encode the vvvv register.

opcodes/ChangeLog:

        * i386-opc.h (SWAP_SOURCES): Dropped.
        (NO_DEFAULT_MASK): Adjusted the value.
        (ADDR_PREFIX_OP_REG): Ditto.
        (DISTINCT_DEST): Ditto.
        (IMPLICIT_STACK_OP): Ditto.
        (VexVVVV_SRC2): New.
        * i386-opc.tbl: Dropped SwapSources and replaced its VexVVVV
	with Src1VVVV.
	* i386-tbl.h: Regenerated.