Update prefixed attribute for Power10.

Update prefixed attribute for Power10.

This patch creates a new attribute, "maybe_prefixed", which is used to mark
those instructions that may have a prefixed form. The existing "prefixed"
attribute is now used to mark all instructions that are prefixed form.

2021-03-31  Pat Haugen  <pthaugen@linux.ibm.com>

gcc/
	PR target/99133
	* config/rs6000/altivec.md (xxspltiw_v4si, xxspltiw_v4sf_inst,
	xxspltidp_v2df_inst, xxsplti32dx_v4si_inst, xxsplti32dx_v4sf_inst,
	xxblend_<mode>, xxpermx_inst, xxeval): Mark prefixed.
	* config/rs6000/mma.md (mma_<vvi4i4i8>, mma_<avvi4i4i8>,
	mma_<vvi4i4i2>, mma_<avvi4i4i2>, mma_<vvi4i4>, mma_<avvi4i4>,
	mma_<pvi4i2>, mma_<apvi4i2>, mma_<vvi4i4i4>, mma_<avvi4i4i4>):
	Likewise.
	* config/rs6000/rs6000.c (rs6000_final_prescan_insn): Adjust test.
	* config/rs6000/rs6000.md (define_attr "maybe_prefixed"): New.
	(define_attr "prefixed"): Update initializer.