[llvm] [ARM][ConstantIslands] Correct MinNoSplitDisp calculation (PR #114590)

[Renato Golin via llvm-commits]

rengolin wrote:

Adding some Arm folks, as constant island isn't as trivial as it sounds. Your description makes sense and the code could be interpreted as "someone forgot to subtract the start offset", but I want to make sure this is what we think it is.

Also, needs tests. I imagine a "large enough" function IR to trigger the split but a small enough BB IR to not be necessary, and show that there are no errors or crashes.

https://github.com/llvm/llvm-project/pull/114590