rs6000: Simplify some conditions or code related to TARGET_DIRECT_MOVE

When I was making a patch to rework TARGET_P8_VECTOR, I
noticed that there are some redundant checks and dead code
related to TARGET_DIRECT_MOVE, so I made this patch as one
separated preparatory patch, it consists of:
  - Check either TARGET_DIRECT_MOVE or TARGET_P8_VECTOR only
    according to the context, rather than checking both of
    them since they are actually the same (TARGET_DIRECT_MOVE
    is defined as TARGET_P8_VECTOR).
  - Simplify TARGET_VSX && TARGET_DIRECT_MOVE as
    TARGET_DIRECT_MOVE since direct move ensures VSX enabled.
  - Replace some TARGET_POWERPC64 && TARGET_DIRECT_MOVE as
    TARGET_DIRECT_MOVE_64BIT to simplify it.
  - Remove some dead code guarded with TARGET_DIRECT_MOVE
    but the condition never holds here.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (rs6000_option_override_internal): Simplify
	TARGET_P8_VECTOR && TARGET_DIRECT_MOVE as TARGET_P8_VECTOR.
	(rs6000_output_move_128bit): Simplify TARGET_VSX && TARGET_DIRECT_MOVE
	as TARGET_DIRECT_MOVE.
	* config/rs6000/rs6000.h (TARGET_XSCVDPSPN): Simplify conditions
	TARGET_DIRECT_MOVE || TARGET_P8_VECTOR as TARGET_P8_VECTOR.
	(TARGET_XSCVSPDPN): Likewise.
	(TARGET_DIRECT_MOVE_128): Simplify TARGET_DIRECT_MOVE &&
	TARGET_POWERPC64 as TARGET_DIRECT_MOVE_64BIT.
	(TARGET_VEXTRACTUB): Likewise.
	(TARGET_DIRECT_MOVE_64BIT): Simplify TARGET_P8_VECTOR &&
	TARGET_DIRECT_MOVE as TARGET_DIRECT_MOVE.
	* config/rs6000/rs6000.md (signbit<mode>2, @signbit<mode>2_dm,
	*signbit<mode>2_dm_mem, floatsi<mode>2_lfiwax,
	floatsi<SFDF:mode>2_lfiwax_<QHI:mode>_mem_zext,
	floatunssi<mode>2_lfiwzx, float<QHI:mode><SFDF:mode>2,
	*float<QHI:mode><SFDF:mode>2_internal, floatuns<QHI:mode><SFDF:mode>2,
	*floatuns<QHI:mode><SFDF:mode>2_internal, p8_mtvsrd_v16qidi2,
	p8_mtvsrd_df, p8_xxpermdi_<mode>, reload_vsx_from_gpr<mode>,
	p8_mtvsrd_sf, reload_vsx_from_gprsf, p8_mfvsrd_3_<mode>,
	reload_gpr_from_vsx<mode>, reload_gpr_from_vsxsf, unpack<mode>_dm):
	Simplify TARGET_DIRECT_MOVE && TARGET_POWERPC64 as
	TARGET_DIRECT_MOVE_64BIT.
	(unpack<mode>_nodm): Simplify !TARGET_DIRECT_MOVE || !TARGET_POWERPC64
	as !TARGET_DIRECT_MOVE_64BIT.
	(fix_trunc<mode>si2, fix_trunc<mode>si2_stfiwx,
	fix_trunc<mode>si2_internal): Simplify TARGET_P8_VECTOR &&
	TARGET_DIRECT_MOVE as TARGET_DIRECT_MOVE.
	(fix_trunc<mode>si2_stfiwx, fixuns_trunc<mode>si2_stfiwx): Remove some
	dead code as the guard TARGET_DIRECT_MOVE there never holds.
	(fixuns_trunc<mode>si2_stfiwx): Change TARGET_P8_VECTOR with
	TARGET_DIRECT_MOVE which is a better fit.
	* config/rs6000/vsx.md (define_peephole2 for SFmode in GPR): Simplify
	TARGET_DIRECT_MOVE && TARGET_POWERPC64 as TARGET_DIRECT_MOVE_64BIT.