[testsuite] Emit 'warning' instead of 'error' diagnostics for 'dg-optimized', 'dg...

[testsuite] Emit 'warning' instead of 'error' diagnostics for 'dg-optimized', 'dg-missed'

The diagnostics produced by 'dg-optimized', 'dg-missed' aren't error
diagnostics (fatal, meaning: causes compilation to fail) but rather warning
diagnostics (non-fatal, doesn't cause compilation to fail).  Thus, same as
'dg-message', these should use 'saved-dg-warning' instead of 'saved-dg-error',
which then prints: "(test for *warnings*, line [...]) instead of currently:
"(test for *errors*, line [...])".

This is a small bug-fix for commit ed2d9d3720adef3a260b8a55e17e744352a901fc
"dumpfile.c: use prefixes other than 'note: ' for
MSG_{OPTIMIZED_LOCATIONS|MISSED_OPTIMIZATION}", which added 'dg-optimized',
'dg-missed'.

	gcc/testsuite/
	* lib/gcc-dg.exp (dg-optimized, dg-missed): Use 'saved-dg-warning'
	instead of 'saved-dg-error'.