aarch64: Rename some SME iterators

This patch just renames the iterators SME_READ and SME_WRITE to
SME_READ_HV and SME_WRITE_HV, to distinguish them from other forms
of ZA read and write.

gcc/
	* config/aarch64/iterators.md (SME_READ): Rename to...
	(SME_READ_HV): ...this.
	(SME_WRITE): Rename to...
	(SME_WRITE_HV): ...this.
	* config/aarch64/aarch64-sme.md: Update accordingly.