Move XXEVAL from altivec.md to vsx.md.

Move XXEVAL from altivec.md to vsx.md.

This patch moves the XXEVAL support from altivec.md to vsx.md.  Given
the instruction support all VSX registers, vsx.md is a more proper place
for this instruction.

gcc/
2021-04-13  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/altivec.md (UNSPEC_XXEVAL): Move to vsx.md.
	(xxeval): Move to vsx.md.
	* config/rs6000/vsx.md (UNSPEC_XXEVAL): Move from altivec.md.
	(xxeval): Move from altivec.md.