RFR: Loom checked stack chunk casts [v3]

[Stefan Karlsson]

> Replace raw oop to stackChunkOops casts with calls to a ::cast function which asserts that the oops is a stackChunkOop. This is similar to what we do for other types like *Klass, *CollectedHeap, *Thread, ...
> 
> Given that we assert when converting oops to stackChunkOops, I've also removed the redundant is_stackChunk asserts.

Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:

  Allow nullptr in the cast function

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Changes:
  - all: https://git.openjdk.java.net/loom/pull/119/files
  - new: https://git.openjdk.java.net/loom/pull/119/files/ca4623c6..ff50caeb

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=loom&pr=119&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=loom&pr=119&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/loom/pull/119.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/119/head:pull/119

PR: https://git.openjdk.java.net/loom/pull/119