[foreign-memaccess+abi] RFR: Simplify code now that bitSize is always aligned to byte boundaries [v2]

[Per Minborg]

> This PR suggests some simplifications now that layouts are always aligned to byte boundaries. 
> 
> The PR also contains some other simplifications/cleanups.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Update after comments

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

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/796/files
  - new: https://git.openjdk.org/panama-foreign/pull/796/files/7f29ba43..560fdf7d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=796&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=796&range=00-01

  Stats: 5 lines in 3 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/796.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/796/head:pull/796

PR: https://git.openjdk.org/panama-foreign/pull/796