[NVPTX] Fix the error in a pattern match in v4i8 comparisons. (#81308)

The replacement should've had BFE() as the arguments for the comparison,
not the source register.

While at that, tighten the patterns a bit, and expand them to cover
variants with immediate arguments. Also change the default lowering of
bfe() to use unsigned variant, so the value of the upper bits is
predictable.