x86: Update stack alignment only if stack is used

x86: Update stack alignment only if stack is used

The same hard register may be used for both stack and non-stack accesses.
Update ix86_find_max_used_stack_alignment to adjust stack alignment only
when a hard register used for a stack access is defined in a basic block
that dominates the block where it is used.

gcc/

	PR target/124098
	* config/i386/i386.cc (ix86_access_stack_p): New.
	(ix86_find_max_used_stack_alignment): Update stack alignment only
	if stack is used.

gcc/testsuite/

	PR target/124098
	* gcc.target/i386/pr124098.c: New test.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>