[NFC] AST: Optimize GenericSignatureImpl::getInnermostGenericParams

[AnthonyLatsis]

Adding a fast path that enables us to simplify the successive loop.

Judging by the abundance of BoundGenericType::getGenericArgs usage, we are going to have to call GenericSignatureImpl::getInnermostGenericParams much more often once we start storing substitution maps in bound generic type nodes, so this is ensuring the latter is as fast as possible without caching.

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For #31895

[AnthonyLatsis]

@swift-ci please smoke test

[AnthonyLatsis]

@CodaFi ping

[CodaFi]

After sitting down and reasoning about this transformation on paper, I think we're good here. There may be some worrying code that slips through the many, many assertions we have in the signature formation code that maintains the depth ordering invariant that allows this code to function, but we'll worry about that bridge when we cross it.

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