RFR: Restore stricter assertions in c1_LIR_x86.cpp

[Roman Kennke]

This change reverts some changes against upstream that we introduced
back when we added barriers to update connection matrix. Then we relaxed
some assertions in C1. Apparently, the need for this has since
disappeared, so let's revert to stricter assertions.

Testing: tier3_gc_shenandoah (fastdebug) and specjvm with c1+connected
heuristics (fastdebug)

http://cr.openjdk.java.net/~rkennke/revert-c1fluff/webrev.00/

Ok?

Roman