Fix tbaa annotation on union selector bytes inside of structs (#54604)

We currently cause a alias analysis contradiction by saying that the
unionselbytes are on the stack, even if they are on a struct. LLVM is
then able to figure out that we giving it a impossible alias situation
(the object doesn't alias itself) and triggers UB.

https://godbolt.org/z/ssEKMzsPf

We may want to do a benchmarks run on this to see if anything too
critical hasn't regressed.

Fixes #54599

---------

Co-authored-by: Cody Tapscott <84105208+topolarity@users.noreply.github.com>

Author: gbaraldi

src/cgutils.cpp

@@ -2806,7 +2806,8 @@ static MDNode *best_field_tbaa(jl_codectx_t &ctx, const jl_cgval_t &strct, jl_da
         }
     }
     if (strct.V && jl_field_isconst(jt, idx) && isLoadFromConstGV(strct.V))
-        return ctx.tbaa().tbaa_const;
+        return ctx.tbaa().tbaa_const; //TODO: it seems odd to have a field with a tbaa that doesn't alias it's containing struct's tbaa
+                                      //Does the fact that this is marked as constant make this fine?
     return tbaa;
 }
 
@@ -2903,7 +2904,7 @@ static jl_cgval_t emit_getfield_knownidx(jl_codectx_t &ctx, const jl_cgval_t &st
             else {
                 ptindex = emit_struct_gep(ctx, cast<StructType>(lt), staddr, byte_offset + fsz1);
             }
-            auto val = emit_unionload(ctx, addr, ptindex, jfty, fsz, al, tbaa, !jl_field_isconst(jt, idx), union_max, ctx.tbaa().tbaa_unionselbyte);
+            auto val = emit_unionload(ctx, addr, ptindex, jfty, fsz, al, tbaa, !jl_field_isconst(jt, idx), union_max, strct.tbaa);
             if (val.V && val.V != addr) {
                 setNameWithField(ctx.emission_context, val.V, get_objname, jt, idx, Twine());
             }

test/compiler/codegen.jl

@@ -938,3 +938,12 @@ BigStructAnyInt() = BigStructAnyInt((Union{Base.inferencebarrier(Float64), Int}=
 @test egal_any54109(Torture1_54109(), Torture1_54109())
 @test egal_any54109(Torture2_54109(), Torture2_54109())
 @test !egal_any54109(Torture1_54109(), Torture1_54109((DefaultOr54109(2.0, false) for i = 1:897)...))
+
+function foo54599()
+    pkgid = Base.identify_package("Test")
+    println(devnull,pkgid)
+    println(devnull, pkgid.uuid)
+    pkgid.uuid
+end
+
+@test foo54599() !== nothing