[PM] Teach the always inlining test case to be much more strict about

whether functions are removed, and fix the new PM's always inliner to
actually pass this test.

Without this, the new PM's always inliner leaves all the functions
kicking around which won't work out very well given the semantics of
always inline.

Doing this really highlights how frustrating the current alwaysinline
semantic contract is though -- why can we put it on *external*
functions, etc?

Also I've added a number of tricky and interesting test cases for
removing functions with the always inliner. There is one remaining case
not handled -- fully removing comdats -- and I've left a FIXME about
this.

llvm-svn: 290457