RFR: 8255301: Common and strengthen the code in ciMemberName and ciMethodHandle

[Aleksey Shipilev]

There is a TODO item in their `get_vm_target`-s. We can clean those up. It looks to me the caller code does not handle `NULL` result well, which means we better `fatal` ourselves before exposing `NULL` to callers and `SEGV`-ing there.

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Commit messages:
 - Rename methods a bit
 - 8255301: Common and strengthen the code in ciMemberName and ciMethodHandle

Changes: https://git.openjdk.java.net/jdk/pull/825/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=825&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255301
  Stats: 21 lines in 3 files changed: 8 ins; 6 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/825.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/825/head:pull/825

PR: https://git.openjdk.java.net/jdk/pull/825