[PowerPC] Support test data class intrinsic of 128-bit float

[qiucf]

Summary

We've exploited test data class instructions for f32 f64 v2f64` and v4f32. This patch unifies the LLVM intrinsic into the same ppc_test_data_class and add support for 128-bit precision float values using xststdcqp.

Vector versions of the intrinsic can't be unified because they return vector int instead of int.

[shchenz]

The error message needs to be updated too:

def err_ppc_invalid_test_data_class_type : Error<
  "expected a 'float' or 'double' for the first argument">;

And can we add a test case for ibm float128 in the clang?

[qiucf]

Change error message regarding __float128.

[shchenz]

LGTM. Thanks.