RFR: Loom checked stack chunk casts [v2]

[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:

  Don't cast nullptr to oop

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

Changes:
  - all: https://git.openjdk.java.net/loom/pull/119/files
  - new: https://git.openjdk.java.net/loom/pull/119/files/adefd721..ca4623c6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=loom&pr=119&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=loom&pr=119&range=00-01

  Stats: 17 lines in 3 files changed: 0 ins; 0 del; 17 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