RFR: 8286378: Address possibly lossy conversions in java.base

[Roger Riggs]

PR#8599 8244681: proposes to add compiler warnings for possible lossy conversions
>From the CSR:

"If the type of the right-hand operand of a compound assignment is not assignment compatible with the type of the variable, a cast is implied and possible lossy conversion may silently occur. While similar situations are resolved as compilation errors for primitive assignments, there are no similar rules defined for compound assignments."

This PR anticipates the warnings and adds explicit casts to replace the implicit casts.
In most cases, the cast matches the type of the right-hand side to type of the compound assignment.
Since these casts are truncating the value, there might be a different refactoring that avoids the cast
but a direct replacement was chosen here.

(Advise and suggestions will inform similar updates to other OpenJDK modules).

-------------

Commit messages:
 - 8286378: Address possibly lossy conversions in java.base

Changes: https://git.openjdk.java.net/jdk/pull/8642/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8642&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286378
  Stats: 57 lines in 33 files changed: 1 ins; 3 del; 53 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8642.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8642/head:pull/8642

PR: https://git.openjdk.java.net/jdk/pull/8642