RFR: 8340708: Optimize StackMapGenerator::processMethod

[Shaojin Wen]

A small optimization of processMethod, by using local variables and extracting exception methods, reduces codeSize from 326 to 286

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

Commit messages:
 - Update src/java.base/share/classes/jdk/internal/classfile/impl/StackMapGenerator.java
 - optimize processMethod

Changes: https://git.openjdk.org/jdk/pull/21137/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21137&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340708
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21137.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21137/head:pull/21137

PR: https://git.openjdk.org/jdk/pull/21137