dce: Use FOR_EACH_IMM_USE_FAST instead of FOR_EACH_IMM_USE_STMT in simple_dce_from_wo...

dce: Use FOR_EACH_IMM_USE_FAST instead of FOR_EACH_IMM_USE_STMT in simple_dce_from_worklist

When I wrote this code I forgot about FOR_EACH_IMM_USE_FAST. Since this code does not change
any of the uses inside of the loop using FOR_EACH_IMM_USE_FAST will be slightly faster.

Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-ssa-dce.cc (simple_dce_from_worklist): Use FOR_EACH_IMM_USE_FAST instead of
	FOR_EACH_IMM_USE_STMT.

Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>