aarch64: Add missing %z prefixes to LDP/STP patterns

aarch64: Add missing %z prefixes to LDP/STP patterns

For LDP/STP Q, the memory operand might not be valid for "m",
so we need to use %z<N> instead of %<N> in the asm template.
This patch does that for all Ump LDP/STP patterns, regardless
of whether it's strictly needed.

This is needed to unbreak bootstrap.

2020-08-05  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.md (load_pair_sw_<SX:mode><SX2:mode>)
	(load_pair_dw_<DX:mode><DX2:mode>, load_pair_dw_tftf)
	(store_pair_sw_<SX:mode><SX2:mode>)
	(store_pair_dw_<DX:mode><DX2:mode>, store_pair_dw_tftf)
	(*load_pair_extendsidi2_aarch64)
	(*load_pair_zero_extendsidi2_aarch64): Use %z for the memory operand.
	* config/aarch64/aarch64-simd.md (load_pair<DREG:mode><DREG2:mode>)
	(vec_store_pair<DREG:mode><DREG2:mode>, load_pair<VQ:mode><VQ2:mode>)
	(vec_store_pair<VQ:mode><VQ2:mode>): Likewise.