Implement new RTL optimizations pass: fold-mem-offsets

This is a new RTL pass that tries to optimize memory offset calculations
by moving them from add immediate instructions to the memory loads/stores.
For example it can transform this:

  addi t4,sp,16
  add  t2,a6,t4
  shl  t3,t2,1
  ld   a2,0(t3)
  addi a2,1
  sd   a2,8(t2)

into the following (one instruction less):

  add  t2,a6,sp
  shl  t3,t2,1
  ld   a2,32(t3)
  addi a2,1
  sd   a2,24(t2)

Although there are places where this is done already, this pass is more
powerful and can handle the more difficult cases that are currently not
optimized. Also, it runs late enough and can optimize away unnecessary
stack pointer calculations.

gcc/ChangeLog:

	* Makefile.in: Add fold-mem-offsets.o.
	* passes.def: Schedule a new pass.
	* tree-pass.h (make_pass_fold_mem_offsets): Declare.
	* common.opt: New options.
	* doc/invoke.texi: Document new option.
	* fold-mem-offsets.cc: New file.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/fold-mem-offsets-1.c: New test.
	* gcc.target/riscv/fold-mem-offsets-2.c: New test.
	* gcc.target/riscv/fold-mem-offsets-3.c: New test.
	* gcc.target/i386/pr52146.c: Adjust expected output.

Signed-off-by: Manolis Tsamis <manolis.tsamis@vrull.eu>

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