As pointed out by Richard in PR63155.  It speeds up the testcase a few %.

2018-10-08  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR middle-end/63155
	* gimple-ssa-backprop.c (backprop::intersect_uses): Use
	FOR_EACH_IMM_USE_FAST instead of FOR_EACH_IMM_USE_STMT.

From-SVN: r264941