RFR: 8282792: Loom: Fix optimized build after commit 4d2710

[Hao Sun]

In commit 4d2710[1], one invocation of function print_frames() is moved
from condition `!PRODUCT` to condition `ASSERT`. As a result, all the
usages are now under condition `ASSERT`, leading to the compilation
warning "unused-function" in optimized VM build.

Hence, in this patch, we move the defintion of function print_frames()
to condition `ASSERT`.

[1] https://github.com/openjdk/loom/commit/4d2710#diff-74c5a41cb3476b29151b4a13732cf4756879de0aa1b1d8c4d73c38e570413fcfR2851

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Commit messages:
 - 8282792: Loom: Fix optimized build after commit 4d2710

Changes: https://git.openjdk.java.net/loom/pull/98/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=98&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282792
  Stats: 50 lines in 1 file changed: 25 ins; 25 del; 0 mod
  Patch: https://git.openjdk.java.net/loom/pull/98.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/98/head:pull/98

PR: https://git.openjdk.java.net/loom/pull/98