Encode type-/layout- vs. region-based alias info separately in LLVM IR

[topolarity]

Currently we redundantly encode region information in both !tbaa and !alias.scope metadata for LLVM.

We'd like to separate these so that TBAA is only used to encode the layout-/type-based non-aliasing information, and !alias.scope is used just for the region-based information.

For reference, here's the existing TBAA hierarchy:

julia/src/codegen.cpp

Lines 351 to 375 in 5f7bfc0

	struct jl_tbaacache_t {
	// type-based alias analysis nodes. Indentation of comments indicates hierarchy.
	MDNode *tbaa_root; // Everything
	MDNode *tbaa_gcframe; // GC frame
	// LLVM should have enough info for alias analysis of non-gcframe stack slot
	// this is mainly a place holder for `jl_cgval_t::tbaa`
	MDNode *tbaa_stack; // stack slot
	MDNode *tbaa_unionselbyte; // a selector byte in isbits Union struct fields
	MDNode *tbaa_data; // Any user data that `pointerset/ref` are allowed to alias
	MDNode *tbaa_binding; // jl_binding_t::value
	MDNode *tbaa_value; // jl_value_t, that is not jl_array_t or jl_genericmemory_t
	MDNode *tbaa_mutab; // mutable type
	MDNode *tbaa_datatype; // datatype
	MDNode *tbaa_immut; // immutable type
	MDNode *tbaa_ptrarraybuf; // Data in an array of boxed values
	MDNode *tbaa_arraybuf; // Data in an array of POD
	MDNode *tbaa_array; // jl_array_t or jl_genericmemory_t
	MDNode *tbaa_arrayptr; // The pointer inside a jl_array_t (to memoryref)
	MDNode *tbaa_arraysize; // A size in a jl_array_t
	MDNode *tbaa_arrayselbyte; // a selector byte in a isbits Union jl_genericmemory_t
	MDNode *tbaa_memoryptr; // The pointer inside a jl_genericmemory_t
	MDNode *tbaa_memorylen; // The length in a jl_genericmemory_t
	MDNode *tbaa_memoryown; // The owner in a foreign jl_genericmemory_t
	MDNode *tbaa_const; // Memory that is immutable by the time LLVM can see it
	bool initialized;

The first step is probably to (1) re-factor the code to stop using ::fromTBAA. Instead, the region information should be passed around as a separate piece of aliasing-related information. Most likely that means creating a jl_aliasinfo_t much earlier, and updating jl_cgval_t to carry it instead of just TBAA metadata. Then, (2) remove tbaa_gcframe, tbaa_stack, tbaa_const, and tbaa_data from the TBAA hierarchy.

Afterwards, an excellent follow-up will be to expand the TBAA hierarchy to encode a much broader set of types into it, including ideally user-defined structs.