IBM Z: Get rid of vpdi unspec

IBM Z: Get rid of vpdi unspec

The patch gets rid of the unspec used for the vector permute double
immediate instruction and replaces it with generic rtx.

gcc/ChangeLog:

	* config/s390/s390.md (UNSPEC_VEC_PERMI): Remove constant
	definition.
	* config/s390/vector.md (*vpdi1<mode>, *vpdi4<mode>): New pattern
	definitions.
	* config/s390/vx-builtins.md (*vec_permi<mode>): Emit generic rtx
	instead of an unspec.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/zvector/vec-permi.c: Removed.
	* gcc.target/s390/zvector/vec_permi.c: New test.