[mlir][LLVM] Fix unsupported FP lowering in `VectorConvertToLLVMPattern` (#166513)
Fixes a bug in `VectorConvertToLLVMPattern`, which converted operations
with unsupported FP types. E.g., `arith.addf ... : f4E2M1FN` was lowered
to `llvm.fadd ... : i4`, which does not verify. There are a few more
patterns that have the same bug. Those will be fixed in follow-up PRs.
This commit is in preparation of adding an `APFloat`-based lowering for
`arith` operations with unsupported floating-point types.