PR tree-optimization/98335: Improvements to DSE's compute_trims.

This patch is the main middle-end piece of a fix for PR tree-opt/98335,
which is a code-quality regression affecting mainline.  The issue occurs
in DSE's (dead store elimination's) compute_trims function that determines
where a store to memory can be trimmed.  In the testcase given in the
PR, this function notices that the first byte of a DImode store is dead,
and replaces the 8-byte store at (aligned) offset zero, with a 7-byte store
at (unaligned) offset one.  Most architectures can store a power-of-two
bytes (up to a maximum) in single instruction, so writing 7 bytes requires
more instructions than writing 8 bytes.  This patch follows Jakub Jelinek's
suggestion in comment 5, that compute_trims needs improved heuristics.

On x86_64-pc-linux-gnu with -O2 the new test case in the PR goes from:

        movl    $0, -24(%rsp)
        movabsq $72057594037927935, %rdx
        movl    $0, -21(%rsp)
        andq    -24(%rsp), %rdx
        movq    %rdx, %rax
        salq    $8, %rax
        movb    c(%rip), %al
        ret

to

        xorl    %eax, %eax
        movb    c(%rip), %al
        ret

2022-03-11  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	PR tree-optimization/98335
	* builtins.cc (get_object_alignment_2): Export.
	* builtins.h (get_object_alignment_2): Likewise.
	* tree-ssa-alias.cc (ao_ref_alignment): New.
	* tree-ssa-alias.h (ao_ref_alignment): Declare.

	* tree-ssa-dse.cc (compute_trims): Improve logic deciding whether
	to align head/tail, writing more bytes but using fewer store insns.

gcc/testsuite/ChangeLog
	PR tree-optimization/98335
	* g++.dg/pr98335.C: New test case.
	* gcc.dg/pr86010.c: New test case.
	* gcc.dg/pr86010-2.c: New test case.