RFR: 8286391: Address possibly lossy conversions in jdk.compiler

[Adam Sotona]

This is a part of addressing of all possibly lossy conversions in compound assignments across JDK sources.
Two cases have been found in jdk.compiler and they are addressed by this patch.
One case in ClassWriter is resolved by changing local variable from char to int type to avoid multiple implicit and explicit conversions.
Second case in Code is resolved by making the implicit conversion from int to char explicit.

Please review.

Thanks,
Adam

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Commit messages:
 - 8286391: Address possibly lossy conversions in jdk.compiler

Changes: https://git.openjdk.java.net/jdk/pull/8652/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8652&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286391
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8652.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8652/head:pull/8652

PR: https://git.openjdk.java.net/jdk/pull/8652