A '*[N]T' receiver in an index expression reached LLVM emission with an
unresolved element type and tripped the panic sentinel — no read or
write spelling worked. ptrToArrayElem on Lowering recognises the shape;
the index READ path GEPs the pointee array through the pointer value
and loads the element; the write / compound-assign / lvalue /
addr-of-element paths and the expression typer resolve the element type
through the same helper (their GEP machinery already handled a pointer
base). Kept out of getElementType so slice paths don't half-accept a
raw pointer base.
Regression: examples/0176 (read, write, compound, element ptr + deref).
Pre-existing (plain locals repro it); found pinning @K reads for
PLAN-CONST-AGG step 1, which is now blocked on it. No deref spelling
works: p[2] hits the unresolved-type tripwire, (*p)[2] doesn't parse.
