[llvm] [BOLT][AArch64] Check Last Element Instead of Returning `nullptr` in `lookupStubFromGroup` (PR #114015)

[Paschalis Mpeis via llvm-commits]

paschalis-mpeis wrote:

Sharing some stats based on a follow-up investigation I did as part of issue #99848
I saw 30% reduction of local cold stubs in LongJmp in the issue's complex [binary](https://github.com/llvm/llvm-project/issues/99848#issue-2422296515).

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**Details:**
When applying this patch and having the now-merged #96609 fix on, this case was encountered 118 times.
All 118 cases were in range and all concerned local cold stubs.

This caused a total of 388 cold stubs to be created, 121 down from 509 (which is just #96609, without #114015; see table [here](https://github.com/llvm/llvm-project/issues/99848#issuecomment-2454647053) at section 3). 

The extra 3 stubs that were not created (ie the remainder of 121-118), were local cold stubs that happen to remain in the range of [PCOffset](https://github.com/llvm/llvm-project/blob/03948882d3bac33cf71a47df1c7ee0f87aad9fc2/bolt/lib/Passes/LongJmp.cpp#L146), probably due to the overall reduction of stubs.


https://github.com/llvm/llvm-project/pull/114015