[VP] Vector predicated vector splice intrinsic

This patch introduces the vector-predicated version of the
experimental_vector_splice intrinsic [1] at the IR level. It considers
the active vector length for both vectors and and uses a vector mask to
disable certain lanes in the result.

[1] https://reviews.llvm.org/D94708

Change originally authored by Vineet Kumar <vineet.kumar@bsc.es>

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D103898