codegen: fix bitcast of union values with inline_roots (#61027)
## Summary
- When a union type contains members with mixed GC-pointer and
non-pointer fields, codegen uses "split representation"
(`inline_roots`). `generic_bitcast` called
`ispointer()`/`data_pointer()` which assert `inline_roots.empty()`,
causing an assertion failure.
- Handle the `inline_roots` case by loading directly from `v.V` before
falling through to `ispointer()`. The dynamic runtime checks
(isprimitivetype + size match) already guarantee only primitive types
reach the load.
- Discovered via LinearSolve's KLU extension, which bitcasts a union
value containing structs with Vector fields.
## Test plan
- Added test in `test/intrinsics.jl` that exercises
`Core.Intrinsics.bitcast` on a union containing a struct with GC-pointer
fields (split representation).
This PR was written with the assistance of generative AI (Claude).
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
