RFR: 8255441: Cleanup ciEnv/jvmciEnv::lookup_method-s

[Aleksey Shipilev]

Static analysis complains there is a potentially uninitialized `dest_method` after the switch, oblivious of `ShouldNotReachHere()`. This can be cleaned up along with related code.

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Commit messages:
 - 8255441: Cleanup ciEnv/jvmciEnv::lookup_method-s

Changes: https://git.openjdk.java.net/jdk/pull/875/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=875&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255441
  Stats: 55 lines in 2 files changed: 8 ins; 19 del; 28 mod
  Patch: https://git.openjdk.java.net/jdk/pull/875.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/875/head:pull/875

PR: https://git.openjdk.java.net/jdk/pull/875