RFR: 8215473: [lworld] Support for substitutability checks in C2

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8215473/webrev.00/

This gives the option of turning acmp into a substitutability test in c2
code. The actual test on values is performed by always calling
ValueBootstrapMethods.isSubstitutable().

Given this change doesn't include interpreter support and the code path
for value comparison is not optimized, the main purpose of this change,
at this point, is to enable performance evaluation for workloads that
make no use of values.

Roland.