`arith.index_cast` and `arith.index_castui` accept memref operands (via
`IndexCastTypeConstraint`), but `IndexCastOpLowering::matchAndRewrite`
did not handle this case. When the operand was a memref, the conversion
framework substituted the converted LLVM struct type, and the lowering
incorrectly attempted to emit `llvm.sext`/`llvm.zext`/`llvm.trunc` on a
struct value, producing invalid LLVM IR.
Since LLVM uses opaque pointers, all memrefs with integer or index
element types lower to the same `\!llvm.struct<(ptr, ptr, i64, ...)>`
type, making `arith.index_cast` on memrefs a no-op at the LLVM level.
Add a check that treats the memref case as an identity conversion (same
as the same-bit-width path).
Fixes#92377
Assisted-by: Claude Code
249e871fa4