aarch64: Generalize writeback ldp/stp patterns

aarch64: Generalize writeback ldp/stp patterns

Thus far the writeback forms of ldp/stp have been exclusively used in
prologue and epilogue code for saving/restoring of registers to/from the
stack.

As such, forms of ldp/stp that weren't needed for prologue/epilogue code
weren't supported by the aarch64 backend.  This patch generalizes the
load/store pair writeback patterns to allow:

 - Base registers other than the stack pointer.
 - Modes that weren't previously supported.
 - Combinations of distinct modes provided they have the same size.
 - Pre/post variants that weren't previously needed in prologue/epilogue
   code.

We make quite some effort to avoid a combinatorial explosion in the
number of patterns generated (and those in the source) by making
extensive use of special predicates.

An updated version of the upcoming ldp/stp pass can generate the
writeback forms, so this patch is motivated by that.

This patch doesn't add zero-extending or sign-extending forms of the
writeback patterns; that is left for future work.

gcc/ChangeLog:

	* config/aarch64/aarch64-protos.h (aarch64_ldpstp_operand_mode_p): Declare.
	* config/aarch64/aarch64.cc (aarch64_gen_storewb_pair): Build RTL
	directly instead of invoking named pattern.
	(aarch64_gen_loadwb_pair): Likewise.
	(aarch64_ldpstp_operand_mode_p): New.
	* config/aarch64/aarch64.md (loadwb_pair<GPI:mode>_<P:mode>): Replace with
	...
	(*loadwb_post_pair_<ldst_sz>): ... this. Generalize as described
	in cover letter.
	(loadwb_pair<GPF:mode>_<P:mode>): Delete (superseded by the
	above).
	(*loadwb_post_pair_16): New.
	(*loadwb_pre_pair_<ldst_sz>): New.
	(loadwb_pair<TX:mode>_<P:mode>): Delete.
	(*loadwb_pre_pair_16): New.
	(storewb_pair<GPI:mode>_<P:mode>): Replace with ...
	(*storewb_pre_pair_<ldst_sz>): ... this.  Generalize as
	described in cover letter.
	(*storewb_pre_pair_16): New.
	(storewb_pair<GPF:mode>_<P:mode>): Delete.
	(*storewb_post_pair_<ldst_sz>): New.
	(storewb_pair<TX:mode>_<P:mode>): Delete.
	(*storewb_post_pair_16): New.
	* config/aarch64/predicates.md (aarch64_mem_pair_operator): New.
	(pmode_plus_operator): New.
	(aarch64_ldp_reg_operand): New.
	(aarch64_stp_reg_operand): New.