codegen load/store/union cleanup and fix (#60353)

Working with Claude to try to de-duplicate and simplify/cleanup some of the code here (esp. union_store) that has grown chaotic over time, and fix bugs. Fix#60345, Fix#60355 (and other hypothetical similar issues)

Author: vtjnash

Compiler/test/codegen.jl

@@ -1075,3 +1075,13 @@ let io = IOBuffer()
  str =String(take!(io))
 @testoccursin("julia.write_barrier", str)
 end
+
+# Test phi node codegen for union types with inline roots
+functionunion_phi_inline_roots(x::Bool)
+if x
+return ("Q8", 1)
+else
+return ("Q10", Ref(5))
+end
+end
+@testunion_phi_inline_roots(true) === ("Q8", 1)