Get active source lane predicate from shuffle instruction.

nvvm.shfl.sync.bfly optionally returns a predicate whether source lane was active. Support for this was added to clang in https://reviews.llvm.org/D68892.

Add an optional 'pred' unit attribute to the instruction to return this predicate. Specify this attribute in the partial warp reduction so we don't need to manually compute the predicate.

PiperOrigin-RevId: 275616564