[SYCLLowerIR][GlobalOffset] Avoid erasing live non-GEP/PHI defs

[wenju-he]

Follow-up fix for 30e1d2f, in which collectGlobalOffsetUses walked general def-use chains, not just GEP/PHI.
When removing collected pointer defs, only erase instructions that has no use. Non-GEP/PHI intermediates may still have users.

[wenju-he]

@intel/llvm-gatekeepers please merge, thanks, below fail (#20448) is unrelated and can be reproduced locally without this PR

2025-10-23T23:56:41.1389910Z [ RUN      ] CommandGraphTest.LinearInOrderQueue
2025-10-23T23:56:41.1390519Z /__w/llvm/llvm/src/sycl/unittests/Extensions/CommandGraph/LinearGraphOptimization.cpp:42: Failure
2025-10-23T23:56:41.1390665Z Expected equality of these values:
2025-10-23T23:56:41.1390866Z   GraphImplTest::NumPartitionsInOrder(Impl)
2025-10-23T23:56:41.1391028Z     Which is: 2
2025-10-23T23:56:41.1391156Z   NumLinearChains
2025-10-23T23:56:41.1391259Z     Which is: 3
2025-10-23T23:56:41.1391264Z 
2025-10-23T23:56:41.1391715Z /__w/llvm/llvm/src/sycl/unittests/Extensions/CommandGraph/LinearGraphOptimization.cpp:43: Failure
2025-10-23T23:56:41.1391917Z Expected equality of these values:
2025-10-23T23:56:41.1392114Z   GraphImplTest::NumSyncPoints(Impl)
2025-10-23T23:56:41.1392231Z     Which is: 3
2025-10-23T23:56:41.1392391Z   0
2025-10-23T23:56:41.1392397Z 
2025-10-23T23:56:41.1392595Z [  FAILED  ] CommandGraphTest.LinearInOrderQueue (0 ms)

[steffenlarsen]

Failures seem to correspond to those in post-commit CI.