aarch64, middle-end: Move pair_fusion pass from aarch64 to middle-end

aarch64, middle-end: Move pair_fusion pass from aarch64 to middle-end

Move pair fusion pass from aarch64-ldp-fusion.cc to middle-end
to support multiple targets.

Common infrastructure of load store pair fusion is divided into
target independent and target dependent code.

Target independent code is structured in the following files.
gcc/pair-fusion.h
gcc/pair-fusion.cc

Target independent code is the Generic code with pure virtual
function to interface betwwen target independent and dependent
code.

2024-05-30  Ajit Kumar Agarwal  <aagarwa1@linux.ibm.com>

gcc/ChangeLog:

	* pair-fusion.h: Generic header code for load store pair fusion
	that can be shared across different architectures.
	* pair-fusion.cc: Generic source code implementation for
	load store pair fusion that can be shared across different architectures.
	* Makefile.in: Add new object file pair-fusion.o.
	* config/aarch64/aarch64-ldp-fusion.cc: Delete generic code and move it
	to pair-fusion.cc in the middle-end.
	* config/aarch64/t-aarch64: Add header file dependency on pair-fusion.h.
	Remove unnecessary header file dependency.