RFR: 8209136: [JVMCI] Make sure volatile fields are read as volatile during constant reflection.

[Gilles Duboscq]

Please review this change to the way volatile fields are read in the `ConstantReflectionProvider`.

This should not affect normal java programs (there are no `volatile final` fields), it allows users of the JVMCI API to handle things like volatile stable fields:
for example the compiler might want to constant-fold such field loads in program order and get volatile load ordering for the reads during constant folding.

webrev: http://cr.openjdk.java.net/~gdub/webrev-8209136
issue:  https://bugs.openjdk.java.net/browse/JDK-8209136

Mach5 Testing: hs-tier1,hs-tier2,hs-tier3-graal

 Gilles