8359707: Add classfile modification code to RedefineClassHelper

[coleenp]

I copied this code for another test in the Valhalla repo and thought it would be a good utility function. It might be better written using the Classfile API.
Tested with test.

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Issue

• JDK-8359707: Add classfile modification code to RedefineClassHelper (Enhancement - P4)

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[coleenp]

/label add hotspot-runtime

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[mlbridge]

Webrevs

• 01: Full - Incremental (6bc1ca59)
• 00: Full (3654034f)

[sspitsyn]

This looks reasonable. I've posted one nit suggestion.

[sspitsyn]

Nit: I'd suggest to rename parameters:
oldString => oldClassName
newString => newClassName

Alternatively, it is possible to pass bytecodes buffer instead of class loader and keep oldString and newString as before.

[coleenp]

I wanted this code to also read bytecodes off the disk and not the caller do that either. The code for getBytecodes or very similar is duplicated all over the tests also.

[coleenp]

I can add a byte buffer version though.