re PR middle-end/65958 (-fstack-check breaks alloca on architectures using generic stack checking)

	PR middle-end/65958
	* gimplify.c (struct gimplify_ctx): Turn boolean fields into 1-bit
	fields, add keep_stack and reorder them.
	(gimplify_bind_expr): Save gimplify_ctxp->keep_stack on entry then
	set it to false.  Do not insert a stack save/restore pair if it has
	been set to true by the gimplification of the statements.
	Restore it to the saved value on exit if it is still false.
	(gimplify_vla_decl): Do not set gimplify_ctxp->save_stack here.
	(gimplify_call_expr) <BUILT_IN_ALLOCA[_WITH_ALIGN]>: New case.  Set
	either save_stack or keep_stack depending on CALL_ALLOCA_FOR_VAR_P.
	* doc/extend.texi (Variable Length): Document new behavior.
	* doc/generic.texi (Blocks): Document new handling of VLAs.

From-SVN: r231260