SI-8315 Better debugging facility for ICode

Suffix the phase name to avoid clobbering files.

	% qbin/scalac -Xprint-icode -Xprint:inline -Yinline -Ydead-code sandbox/test.scala 1>/dev/null 2>/dev/null
	% ls *.icode
	A$$anonfun$crash$1_icode.icode		Listt_icode.icode
	A$$anonfun$crash$1_inliner.icode	Listt_inliner.icode
	A_icode.icode				Nill$_icode.icode
	A_inliner.icode				Nill$_inliner.icode
SI-8315 Fix crash in dead code elimination

It was a cache invalidation bug.

We need to mark the Code as touched to invalidate the caches
behind `predecessors` and `successors`.
Merge pull request #3567 from retronym/ticket/8315

SI-8315 Fix crash in dead code elimination