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>
(cherry picked from commit 30542e0)

src/cgutils.cpp

@@ -2740,7 +2740,7 @@ 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

@@ -887,3 +887,12 @@ end
 ex54166 = Union{Missing, Int64}[missing -2; missing -2];
 dims54166 = (1,2)
 @test (minimum(ex54166; dims=dims54166)[1] === missing)
+
+function foo54599()
+    pkgid = Base.identify_package("Test")
+    println(devnull,pkgid)
+    println(devnull, pkgid.uuid)
+    pkgid.uuid
+end
+
+@test foo54599() !== nothing