RFR: 8286312: Stop mixing signed and unsigned types in bit operations

[Leo Korinth]

uint32_t is a much more natural type for bitsIdx.

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Commit messages:
 - 8286312: Do not mix signed and unsigned in bit operations

Changes: https://git.openjdk.java.net/jdk/pull/8581/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8581&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286312
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8581.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8581/head:pull/8581

PR: https://git.openjdk.java.net/jdk/pull/8581