[SYCL] Fix 5_GPU_optimized sample

[againull]

Existing Sample Changes

Description

Currently there is an out-of-bound access in last iteration of the main loop in iso3dfd kernel (5_GPU_optimized sample):

oneAPI-samples/DirectProgramming/C++SYCL/StructuredGrids/guided_iso3dfd_GPUOptimization/src/5_GPU_optimized.cpp

Line 146 in ffc4fce

front[kHalfLength] = prev_acc[idx + kHalfLength * n2n3];

prev_acc is accessed out-of-bounds in the last iteration.
To avoid this, don't prepare unnecessary data for the next iteration if the current iteration is the last one.

Fixes Issue#
CMPLRLLVM-69572

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Verified on
Intel(R) Data Center GPU Max 1550
with gpu driver 25.05.32567.17
OS: Ubuntu 22.04

intel/llvm compiler: intel/llvm@e8c8555

Commands:
clang++ -fsycl 5_GPU_optimized.cpp -o 5_GPU_optimized
./5_GPU_optimized 256 256 256 100 256 16 16 verify

Hangs without the fix, passes verification with the fix.