[PR81611] turn inc-and-use-of-dead-orig into auto-inc

[PR81611] turn inc-and-use-of-dead-orig into auto-inc

When the addressing modes available on the machine don't allow offsets
in addresses, odds are that post-increments will be represented in
trees and RTL as:

  y <= x + 1
  ... *(x) ...
  x <= y

so deal with it by turning such RTL as:

  (set y (plus x n))
  ... (mem x) ...

without intervening uses of y into

  (set y x)
  ... (mem (post_add y n)) ...

so as to create auto-inc addresses that we'd otherwise miss.

for  gcc/ChangeLog

	PR rtl-optimization/81611
	* auto-inc-dec.c (attempt_change): Move dead note from
	mem_insn if it's the next use of regno
	(find_address): Take address use of reg holding
	non-incremented value.  Add parm to limit search to the named
	reg only.
	(merge_in_block): Attempt to use a mem insn that is the next
	use of the original regno.

From-SVN: r258053