[lworld] RFR: 8263566: [lworld] Skip generate_return_value_stubs when InlineTypeReturnedAsFields is disabled

[Yi Yang]

This is a trivial change. When InlineTypeReturnedAsFields is turned off, we can skip generating related stub routines, which would reduce C heap occupancy and start VM faster through slightly.

Testing:
x86: all valhalla related tests with release mode
aarch64: none

Best regards,
Yang

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Commit messages:
 - skip generate_return_value_stubs when InlineTypeReturnedAsFields is

Changes: https://git.openjdk.java.net/valhalla/pull/367/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=367&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263566
  Stats: 10 lines in 2 files changed: 5 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/367.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/367/head:pull/367

PR: https://git.openjdk.java.net/valhalla/pull/367