RFR: 8263615: Cleanup tightly_coupled_allocation

[Nils Eliasson]

Small cleanup of tightly_couple_allocation as a follow up to JDK-8263361. 

1. The slow_region argument isn't used from any context. 
2. The use in LibraryCallKit::inline_string_toBytesU() can never be null since the alloca is directly above. Removing null check and associated code.

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Commit messages:
 - assert to guarantee
 - revert
 - clean up
 - Merge branch 'master' into fix_tightly_alloc
 - Remove redundant arg

Changes: https://git.openjdk.java.net/jdk/pull/3174/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3174&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263615
  Stats: 41 lines in 2 files changed: 9 ins; 18 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3174.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3174/head:pull/3174

PR: https://git.openjdk.java.net/jdk/pull/3174