Rename mffgpr/mftgpr insn types and remove Power6 references.

Rename mffgpr/mftgpr insn types and remove Power6 references.

The following is mostly a mechanical change to rename the mffgpr/mftgpr
insn types to mtvsr/mfvsr to be more clear. It also removes Power6
references to those insn types since we no longer generate those
instructions.

2020-09-14  Pat Haugen  <pthaugen@linux.ibm.com>

gcc/
	* config/rs6000/power10.md (power10-mffgpr, power10-mftgpr): Rename to
	power10-mtvsr/power10-mfvsr.
	* config/rs6000/power6.md (X2F_power6, power6-mftgpr, power6-mffgpr):
	Remove.
	* config/rs6000/power8.md (power8-mffgpr, power8-mftgpr): Rename to
	power8-mtvsr/power8-mfvsr.
	* config/rs6000/power9.md (power9-mffgpr, power9-mftgpr): Rename to
	power9-mtvsr/power9-mfvsr.
	* config/rs6000/rs6000.c (rs6000_adjust_cost): Remove Power6
	TYPE_MFFGPR cases.
	* config/rs6000/rs6000.md (mffgpr, mftgpr, zero_extendsi<mode>2,
	extendsi<mode>2, @signbit<mode>2_dm, lfiwax, lfiwzx, *movsi_internal1,
	movsi_from_sf, *movdi_from_sf_zero_ext, *mov<mode>_internal,
	movsd_hardfloat, movsf_from_si, *mov<mode>_hardfloat64, p8_mtvsrwz,
	p8_mtvsrd_df, p8_mtvsrd_sf, p8_mfvsrd_3_<mode>, *movdi_internal64,
	unpack<mode>_dm): Rename mffgpr/mftgpr to mtvsr/mfvsr.
	* config/rs6000/vsx.md (vsx_mov<mode>_64bit, vsx_extract_<mode>,
	vsx_extract_si, *vsx_extract_<mode>_p8): Likewise.