rs6000: Fix condition of define_expand vec_shr_<mode> [PR100645]

rs6000: Fix condition of define_expand vec_shr_<mode> [PR100645]

PR100645 exposes one latent bug in define_expand vec_shr_<mode>
that the current condition TARGET_ALTIVEC is too loose.  The
mode iterator VEC_L contains a few modes, they are not always
supported as vector mode, VECTOR_UNIT_ALTIVEC_OR_VSX_P should
be used like some other VEC_L usages.

	PR target/100645

gcc/ChangeLog:

	* config/rs6000/vector.md (vec_shr_<mode>): Replace condition
	TARGET_ALTIVEC with VECTOR_UNIT_ALTIVEC_OR_VSX_P.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/pr100645.c: New test.