doc: explicitly say 'lifetime' for DCE

doc: explicitly say 'lifetime' for DCE

Say 'memory lifetime' rather than 'memory life' as lifetime is the more
standard term nowadays (indeed we have e.g. -fno-lifetime-dse).

It's also easier to grep for if someone is looking for the documentation on
where we do that.

gcc/ChangeLog:
	* doc/passes.texi (Dead code elimination): Explicitly say 'lifetime'
	as this has become the standard term for what we're doing here.

Signed-off-by: Sam James <sam@gentoo.org>