builtins: Avoid useless char/short -> int promotions before atomics [PR96176]

builtins: Avoid useless char/short -> int promotions before atomics [PR96176]

As mentioned in the PR, we generate a useless movzbl insn before lock cmpxchg.
The problem is that the builtin for the char/short cases has the arguments
promoted to int and combine gives up, because the instructions have
MEM_VOLATILE_P arguments and recog in that case doesn't recognize anything
when volatile_ok is false, and nothing afterwards optimizes the
(reg:SI a) = (zero_extend:SI (reg:QI a))
... (subreg:QI (reg:SI a) 0) ...

The following patch fixes it at expansion time, we already have a function
that is meant to undo the promotion, so this just adds the very common case
to that.

2020-07-15  Jakub Jelinek  <jakub@redhat.com>

	PR target/96176
	* builtins.c: Include gimple-ssa.h, tree-ssa-live.h and
	tree-outof-ssa.h.
	(expand_expr_force_mode): If exp is a SSA_NAME with different mode
	from MODE and get_gimple_for_ssa_name is a cast from MODE, use the
	cast's rhs.

	* gcc.target/i386/pr96176.c: New test.