RFR: 8307588: HotSpotConstantPool#lookupBootstrapMethodInvocation broken by JDK-8301995

[Josef Eisl]

As a result of the changes for [JDK-8301995](https://bugs.openjdk.org/browse/JDK-8301995), `HotSpotConstantPool#lookupBootstrapMethodIntrospection` is broken as it no longer decodes the correct constant pool index for looking up the bootstrap method invocation for invokedynamic.

This fixes the problem and modifies `TestDynamicConstant` to exercise the code in question.

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Commit messages:
 - Fix HotSpotConstantPool#lookupBootstrapMethodInvocation (JDK-8307588)
 - Test for HotSpotConstantPool#lookupBootstrapMethodInvocation (JDK-8307588)

Changes: https://git.openjdk.org/jdk/pull/13858/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13858&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307588
  Stats: 20 lines in 2 files changed: 3 ins; 13 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/13858.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13858/head:pull/13858

PR: https://git.openjdk.org/jdk/pull/13858