RFR: Various GC cleanups

[Thomas Schatzl]

Please review these cleanups, mostly done by @stefank.

It is probably best to review the commits separately.

Testing: loom-tier1-3

Regards,
  Thomas

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Commit messages:
 - minor additions
 - Move functions to stackChunkOopDesc
 - Harmonize relativize and transform stack chunk functions
 - Small alignment issue
 - Deduplicate relativize_frame
 - Remove duplication of bit index calculations
 - Remove bit_offset use 0 indexing for bits
 - Clean up more assert (
 - Remove casts in iterate_stack

Changes: https://git.openjdk.java.net/loom/pull/123/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=123&range=00
  Stats: 782 lines in 17 files changed: 335 ins; 358 del; 89 mod
  Patch: https://git.openjdk.java.net/loom/pull/123.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/123/head:pull/123

PR: https://git.openjdk.java.net/loom/pull/123