RFR: 8338881: GenShen: Use explicit third temp register for post barrier [v2]

[William Kemper]

> * Use register given as `tmp3` argument instead of "out of the blue" `r3`
> * Simplify logic for post barrier call
> 
> ## Testing
> GHA, internal pipelines

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Use parentheses for comparison operation

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/486/files
  - new: https://git.openjdk.org/shenandoah/pull/486/files/fa7dc0c4..b178e3b8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=486&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=486&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/486.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/486/head:pull/486

PR: https://git.openjdk.org/shenandoah/pull/486