RFR (S) 8212956: [JVCMI] SpeculationReason should maintain identity

[Tom Rodriguez]

http://cr.openjdk.java.net/~never/8212956/webrev
https://bugs.openjdk.java.net/browse/JDK-8212956

The changes for JDK-8205824 introduced a level of indirection in the 
Speculation implementation for JVMCI but lost some identity along the 
way. Speculating on the same SpeculationReason multiple times should 
result in an equivalent Speculation and JavaConstant encoding to avoid 
unnecessary duplication of uncommon traps.  Tested with new unit test 
and graal gates.

tom