dce: Remove FIXME that has not been true for years

dce: Remove FIXME that has not been true for years

This FIXME:
   FIXME: Aggressive mode before PRE doesn't work currently because
	  the dominance info is not invalidated after DCE1.

Has not been true since at least r0-104723-g5ac60b564faa85 which
added a call to calculate_dominance_info. Plus we run agressive mode
before PRE since r0-89162-g11b08ee9118d10 too. And since r0-95499-gb5b8b0ac643d31,
dominance information was required even for non-agressive mode.

Also we have been verifying dominance information is correct and not needing to invalidate
since ssa branch was merged so this comment has been out of date even before it was merged in.

gcc/ChangeLog:

	* tree-ssa-dce.cc (perform_tree_ssa_dce): Remove FIXME note.

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