RFR: 8256365: Clean up vtable initialization code [v3]

[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.

Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:

  Rename is_override to can_be_overridden.

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1236/files
  - new: https://git.openjdk.java.net/jdk/pull/1236/files/364f8195..5f51ef5b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1236&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1236&range=01-02

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 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