RFR: 8256365: Clean up vtable initialization code

[Coleen Phillimore]

I was looking through this code because of JDK-8061949 and want to do some minor cleanups.
1. There's a function in the wrong place (is_override)
2. methodHandles that use mh()->is_native(), with extra (),
3. some methods declared with TRAPS, that don't trap
4. some multi-clause conditionals with confusing formatting
5. extra InstanceKlass::cast() casts
6. some useless asserts
7. and potentially a bug with RedefineClasses where the method being added to the vtable may have been redefined in the constraint verification call. (noreg-hard)

Tested with tier1-3.

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Commit messages:
 - 8256365: Clean up vtable initialization code

Changes: https://git.openjdk.java.net/jdk/pull/1236/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1236&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256365
  Stats: 130 lines in 4 files changed: 33 ins; 39 del; 58 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1236.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1236/head:pull/1236

PR: https://git.openjdk.java.net/jdk/pull/1236