RFR: 8263450: Simplify LambdaForm.useCount

[Claes Redestad]

Simplify useCount and avoid calling lastUseIndex for a small startup win.

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Commit messages:
 - Simplify LambdaForm.useCount

Changes: https://git.openjdk.java.net/jdk/pull/2940/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2940&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263450
  Stats: 16 lines in 1 file changed: 4 ins; 6 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2940.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2940/head:pull/2940

PR: https://git.openjdk.java.net/jdk/pull/2940