AArch64: Add extraRegister to OMRMemoryReference

[knn-k]

This commit adds extraRegister and related code to
OMRMemoryReference for AArch64.
It will be used by UnresolvedSnippet.

Signed-off-by: KONNO Kazuhiro konno@jp.ibm.com

[knn-k]

This is a revisit of #3953.
UnresolvedDataSnippet requires the introduction of modBase register.

[knn-k]

Cancelling this PR.
I am adding the modBase register to J9MemoryReference now. No need to introduce J9_PROJECT_SPECIFIC code to OMRMemoryReference.

[knn-k]

Reopened this PR.

I have to add the _modBase member to OMR::ARM64::MemoryReference instead of J9::ARM64::MemoryReference because:

• Two constructors of OMR::ARM64::MemoryReference call addToOffset().
• addToOffset() is defined in the OMR side. It calls decNodeReferenceCounts().
• decNodeReferenceCounts() needs to call stopUsingRegister(_modBase).
• If I add _modBase to J9::ARM64::MemoryReference, and override the function decNodeReferenceCounts() in J9::ARM64::MemoryReference for adding the code stopUsingRegister(_modBase), that does not work as expected. _modBase is not initialized when the constructor of OMR::ARM64::MemoryReference calls addToOffset().
  "Never call virtual functions during construction or destruction."

[knn-k]

Rebased the code.

[knn-k]

Renamed modBase to extraRegister, and removed the guards with J9_PROJECT_SPECIFIC.
Also changed the title and the commit comment.

[0xdaryl]

I think it would be useful to document what the extra register is for. Any downstream project can use it to associate an additional assigned register to a memory reference that can be used for project-specific purposes. This register is in addition to the base and index registers and isn't directly part of the addressing expression in the memory reference. An example of what this could be used for is to help synthesize unresolved data reference addresses at runtime.

[knn-k]

Added comments in the code on the potential use of the extra register.

[0xdaryl]

@genie-omr build aarch64