[mlir][spirv] `ConvertChainedBitcast` canonicalization can produce wrong code

[Hardcode84]

func.func @convert_bitcast(%arg0 : f64) -> f64 {
  %1 = spirv.Bitcast %arg0 : f64 to i64
  %2 = spirv.Bitcast %1 : i64 to f64
  spirv.ReturnValue %2 : f64
}

error: result type must be different from operand type

[llvmbot]

@llvm/issue-subscribers-mlir-spirv

[antiagainst]

Looks we just need to add a simple fold() case to spirv.Bitcast to fold same type cases.

[antiagainst]

In general the folders in SPIR-V dialect is kinda lacking..