RFR: JDK-8210435: don't add local variable spots if they are DCE'ed by the compiler

[Vicente Romero]

Please review the fix for [1] at [2]. This patch comes from a bug 
reported in amber-dev [3] in the context of the condy-folding project. 
In that discussion it was noted that the same issue can be reproduced in 
vanilla JDK when final local variables are into play.

Thanks,
Vicente

[1] https://bugs.openjdk.java.net/browse/JDK-8210435
[2] http://cr.openjdk.java.net/~vromero/8210435/webrev.00/jdk.dev.patch
[3] 
http://mail.openjdk.java.net/pipermail/amber-dev/2018-September/003407.html