RFR: Loom checked stack chunk casts

[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.

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Commit messages:
 - Remove redundant is_stackChunk asserts
 - Use cast function to convert from oop to stackChunkOop

Changes: https://git.openjdk.java.net/loom/pull/119/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=119&range=00
  Stats: 47 lines in 8 files changed: 10 ins; 16 del; 21 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