[mlir:llvm] `LLVM::ConstantOp` allows mismatched constant value

These are valid instances of `LLVM::ConstantOp` in MLIR:

```llvm
%cst0 = llvm.mlir.constant(1 : i32) : f64
%cst1 = llvm.mlir.constant(1.0 : f64) : f32
```

But they both fail to export. The former triggers an assert `isa<FloatAttr>` and the latter fails to export due to mismatched float size.