RFR: 8341510: Optimize StackMapGenerator::processFieldInstructions

[Shaojin Wen]

A small optimization to reduce CodeSize, codeSize reduced from 164 to 140.

1. Use local currentFrame to avoid multiple getfields
2. Use decStack instead of popStack to reduce array access in popStack
3. Call Util.fieldTypeSymbol to pass in type instead of nameAndType

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Commit messages:
 - optimize processFieldInstructions

Changes: https://git.openjdk.org/jdk/pull/21345/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21345&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341510
  Stats: 14 lines in 4 files changed: 1 ins; 7 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/21345.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21345/head:pull/21345

PR: https://git.openjdk.org/jdk/pull/21345