Skip empty projections in UnDerefer

[tachsin]

Skip empty projections when replaying CopyForDeref chains.

PostAnalysisNormalize can strip an OpaqueCast that was the only
projection, leaving CopyForDeref of a local. Drop elaboration then
panicked in UnDerefer by indexing that empty projection.

Fixes #160800

r? @ghost

LLM disclosure: I used Cursor to investigate #160800 and implement the
UnDerefer change plus a UI regression test. I reviewed the MIR before and
after PostAnalysisNormalize, confirmed the empty-projection panic, and ran
the new test plus related TAIT drop-elaboration tests locally.

This is not pre-arranged with a reviewer. rust-lang LLM policy asks new
contributors to find a reviewer (e.g. #t-compiler/llm-mentoring on Zulip)
before LLM-assisted PRs are assigned. Using r? @ghost until then.

Made with Cursor

[rustbot]

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