[PATCH, rs6000] Cleanup some vstrir define_expand naming inconsistencies

[PATCH, rs6000] Cleanup some vstrir define_expand naming inconsistencies

  This cleans up some of the naming around the vstrir and vstril
instruction definitions, with some cosmetic changes for consistency.
No functional changes.
Regtested just in case, no regressions.

[V2] Used 'direct' instead of 'internal', and cosmetically reworked
the changelog.

gcc/
	* config/rs6000/altivec.md:
	(vstrir_code_<mode>): Rename to...
	(vstrir_direct_<mode>): ... this.
	(vstrir_p_code_<mode>): Rename to...
	(vstrir_p_direct_<mode>): ... this.
	(vstril_code_<mode>): Rename to...
	(vstril_direct_<mode>): ... this.
	(vstril_p_code_<mode>): Rename to...
	(vstril_p_direct_<mode>): ... this.