cfgexpand: Update cache during the original DFS walk

cfgexpand: Update cache during the original DFS walk

This is a small optimization which can improve how many times are need through the update loop.
It can reduce the number of times in the update loop by maybe 1 times.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* cfgexpand.cc (vars_ssa_cache::operator()): Update the cache if the use is already
	has a cache.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>