[FunctionAttrs] Incorrect nonnull inference for non-inbounds GEP

[nikic]

FunctionAttrs incorrectly infers that the following function returns nonnull (https://llvm.godbolt.org/z/49Kbq15xn):

define ptr @test(ptr nonnull %arg) {
  %res = getelementptr i8, ptr %arg, i64 -8
  ret ptr %res
}

This is because the code assumes that the result of a GEP is non-null if the source is non-null here:

llvm-project/llvm/lib/Transforms/IPO/FunctionAttrs.cpp

Line 1189 in d98a785

case Instruction::GetElementPtr:

This is not correct for non-inbounds GEPs.