[SYCL] Defer shadow copy creation in SYCLMemObjT

[0x12CC]

#10334 causes a performance regression since HostPtr can't be reused when it's read-only. This PR fixes the regression by deferring the copy operation to the creation of a writable accessor. It includes following the changes:

• A new SYCLMemObjT::MCreateShadowCopy to defer allocation. When the HostPtr cannot be reused since it's read-only, SYCLMemObjT::handleHostData sets this member to a function that will allocate the shadow copy.
• A new SYCLMemObjT::handleWriteAccessorCreation member function. This function calls SYCLMemObjT::MCreateShadowCopy and updates any existing MAllocaCommands if MUserPtr changed.
• Whenever a writable host or device accessor is created, handleWriteAccessorCreation gets called to ensure that any required memory allocation occurs.

With this change, the allocation and copying overhead occurs during the creation of the first writable accessor. There's no overhead if all of the relevant accessors use sycl::access_mode::read.

[aelovikov-intel]

Could there be any race condition if we'd have two concurrent threads with different read/write properties? IIUC, there will be no UB as long as the actual accesses are all read-only.

[0x12CC]

Could there be any race condition if we'd have two concurrent threads with different read/write properties? IIUC, there will be no UB as long as the actual accesses are all read-only.

That's a good question. I'm not certain if my original implementation could cause race conditions so I've updated it. I moved the shadow copy creation to the Scheduler::GraphBuilder so that it runs within a write lock. This change should prevent any races between different accessors.

[steffenlarsen]

LGTM! @aelovikov-intel - Would you like another look?

[steffenlarsen]

Tiny nit to avoid a bit of indentation. If you prefer the current version I am also okay with sticking with it.

Suggested change

	if (MRecord != nullptr && MUserPtr != InitialUserPtr) {
	for (auto &it : MRecord->MAllocaCommands) {
	if (it->MMemAllocation == InitialUserPtr) {
	it->MMemAllocation = MUserPtr;
	}
	}
	}
	if (!MRecord || MUserPtr == InitialUserPtr)
	return;
	for (auto &it : MRecord->MAllocaCommands)
	if (it->MMemAllocation == InitialUserPtr)
	it->MMemAllocation = MUserPtr;

[aelovikov-intel]

LGTM! @aelovikov-intel - Would you like another look?

I'm good with your review.

@0x12CC , this needs a merge with conflicts resolution.