[KnownFPClass] `G_INTRINSIC_FPTRUNC_ROUND` is incorrectly handled

https://github.com/llvm/llvm-project/blob/89082772bed1ed16c0311b12147a96c35bb063d8/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp#L1579-L1586

`G_INTRINSIC_FPTRUNC_ROUND` is incorrectly handled as an integer-rounding intrinsic instead of a floating point truncation intrinsic.

We would also need to tests for `G_INTRINSIC_FPTRUNC_ROUND`.

Start in llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp at lines 1579-1586 and inspect how G_INTRINSIC_FPTRUNC_ROUND is classified. Add coverage for this intrinsic, verifying that it is handled as floating-point truncation rather than integer rounding. Done means the new tests pass and cover the corrected behavior.

Written by the indexing model from the issue text.