8371783: Refactor GCCause in VM_CollectForMetadataAllocation

[jsikstro]

Hello,

We should improve how we use and set GCCause in VM_CollectForMetadataAllocation to make it more clear when reading the code what is going on.

When constructing a VM_CollectForMetadataAllocation operation, we pass in a GCCause, which is not always the cause that is used, since we might also have to clear soft references as a last resort if earlier collection attemps is not enough to free up memory.

The GCCauseSetter at the start of this operation is also redundant as subsequent calls to initiate a GC set the cause themselves.

Testing:

• Oracle's tier1-3

---

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Issue

• JDK-8371783: Refactor GCCause in VM_CollectForMetadataAllocation (Enhancement - P4)

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