codegen: avoid bad tbaa paths

[vtjnash]

This avoids cases where getMostGenericTBAA could return broken answers.

fix #33590

[Keno]

LGTM in terms of changes, but looks like one of the llvmpasses tests needs to be updated.

[vtjnash]

Oops, I blamed getMostGenericTBAA, but the bad TBAA node was actually emitted on the next line of code (which tried to handle when getMostGenericTBAA fails)

julia/src/intrinsics.cpp

Lines 814 to 819 in 4e31741

	if (ifelse_tbaa == NULL) {
	// LLVM won't return a TBAA result for the root, but mark_julia_struct requires it: make it now
	auto *OffsetNode = ConstantAsMetadata::get(ConstantInt::get(T_int64, 0));
	Metadata *Ops[] = {tbaa_root, tbaa_root, OffsetNode};
	ifelse_tbaa = MDNode::get(jl_LLVMContext, Ops);
	}