RFR(XS): 8213779: Loop opts anti dependent store detection should ignore uncommon trap calls

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8213779/webrev.00/

This is a small improvement to anti dependent store detection during
loop opts: uncommon trap calls don't make any change to the memory state
so shouldn't be considered as stores. Unrelated, this patch removes what
appears to be dead code in ifnode.cpp (an IfNode never has 3 inputs).

Roland.