Make it possible to lower fp8 `tt.splat`.
Make it possible to lower fp8 `tt.splat`.

Before the fix, `tt.splat` was lowered to e.g.
```
%14 = "llvm.mlir.constant"() <{value = 0.000000e+00 : f8E4M3FNUZ}> : () -> f8E4M3FNUZ
```
which LLVM rejected.

Translating the result type through typeConverter is what is done in other similar places. It results in
```
%14 = "llvm.mlir.constant"() <{value = 0.000000e+00 : f8E4M3FNUZ}> : () -> i8
```
and it's accepted by LLVM. During the MLIR to LLVM lowering, the fp8 value is converted to i8 with the correct binary representation.

The `isFloat()` function that is updated happened to have just one caller (in ArithConstantSplatOpConversion)
