RFR: 8297728: Cache invocation type rather than invoker in NamedFunction

[Claes Redestad]

`java.lang.invoke.LambdaForm.NamedFunction` has an `invoker` field acting as a cache, but this field is only used for lambda form interpretation, thus not very performance sensitive. 

Since the invoker is already cached in the method type hierarchy we could replace it with a cache of the invocation type instead. This means cost to retrieve an invoker for lambda form interpretation is only slightly affected (a few ns/op that end up in the noise since interpretation already incur a >50ns/op overhead on even the simplest forms), while speeding up bytecode generation for `LF`s with `NamedFunction`s not holding a resolvedHandle.

Being lazy about the creation of the type is not significantly beneficial right now, but is done in anticipation of future changes that may better defer bytecode spinning for `LambdaForm`s.

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Commit messages:
 - Lazily calculate type
 - Cache invocation type rather than invoker in NamedFunction

Changes: https://git.openjdk.org/jdk/pull/11424/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11424&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297728
  Stats: 19 lines in 1 file changed: 9 ins; 6 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/11424.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11424/head:pull/11424

PR: https://git.openjdk.org/jdk/pull/11424