Remove allocation prefetch experimental code

[0xdaryl]

There was an experimental allocation prefetch strategy implemented more than a decade ago (I think it was mine!) that was never enabled and never went past the experimental stage. It has long been deprecated. Remove it.

Look for code guarded with TR_EnableNewX86PrefetchTLH and remove dead code produced from that exercise. A followup PR in OMR to remove the option is in order as well.

[0xdaryl]

@mw918

[0xdaryl]

TR_EnableNewX86PrefetchTLH is the internal Option flag that enables the feature to be removed. You should assume that any code that asks whether this feature is enabled will receive false as the answer. For example,
https://github.com/eclipse/openj9/blob/a235102acaeb0cf7dee1bd9590bd714b0daf66eb/runtime/compiler/x/codegen/J9TreeEvaluator.cpp#L6342
the code guarded here should be considered to be always enabled and the else code should be considered dead and can be removed. You can fold this if/else statement based on that logic.

Similarly, in AllocPrefetchSnippet.cpp any code that would become active if this option were to be active is now dead. This means removing the TR_X86newPrefetchTLH enum and the assembler helper code that goes along with it.

Repeat this process of assuming the option is false and removing/folding code accordingly for each reference to this feature you find in the code. Once it has been removed from OpenJ9, you can remove the TR_X86newPrefetchTLH Option from OMR.

[0xdaryl]

@jmesyou : please complete this

[0xdaryl]

I asked @BradleyWood to investigate TLH prefetching performance, so this cleanup work could be done as part of that investigation (unless you have a PR imminent for this @jmesyou).