RFR: 8274161: Cleanup redundant casts in java.compiler

[Andrey Turbanov]

Such casts are actually redundant, but they are inserted, because variable is declared with too weak type.

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Commit messages:
 - [PATCH] Cleanup redundant casts, when type is actually known beforehand

Changes: https://git.openjdk.java.net/jdk/pull/5283/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5283&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274161
  Stats: 22 lines in 5 files changed: 0 ins; 0 del; 22 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5283.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5283/head:pull/5283

PR: https://git.openjdk.java.net/jdk/pull/5283