Add support for tt.bitcast on tensor of pointers

[Nullkooland]

1. In TritonToStructured, move tt.bitcast on tensor of pointers to the source scalar pointer tracked by PtrAnalysis, for instance:

%6 = tt.splat %arg0: !tt.ptr<i1> -> tensor<512x!tt.ptr<i1>>
%7 = tt.addptr %6, %offsets : tensor<512x!tt.ptr<i1>>, tensor<512xi32>
%8 = tt.bitcast %7 : tensor<512x!tt.ptr<i1>> -> tensor<512x!tt.ptr<i8>>

to

 %3 = tt.bitcast %arg0 : !tt.ptr<i1> -> !tt.ptr<i8>
 %4 = tts.make_tptr %3 to sizes: [512], strides: [1], offsets: [%offsets], shape: [0], order: [] : <i8> to tensor<512x!tt.ptr<i8>>

2. In TritonToUnstructured, Allow tt.bitcast op in ptr sequence.
3. In TritonPtrToMemref, convert tt.bitcast op on ptr to builtin.unrealized_conversion_cast op on unranked memref, since currently memref dialect does not have a suitable op for "pointer reinterpret cast" semantics.

The whole thing is kind of hacky, maybe we can wait for upstream ptr dialect is fully landed, and rework on the whole pointer bitcast lowering.

[Nullkooland]

Related PR #171 @parsifal-47