rs6000: Fix vector homogeneous aggregates (PR86197)

rs6000: Fix vector homogeneous aggregates (PR86197)

The existing code allows only 4 vectors worth of ieee128 homogeneous
aggregates, but it should be 8.  This happens because at one spot it
is mistakenly qualified as being passed in floating point registers.

	PR target/86197
	* config/rs6000/rs6000.md (rs6000_discover_homogeneous_aggregate): An
	ieee128 argument takes up only one (vector) register, not two (floating
	point) registers.

From-SVN: r261738