[IR]Add NumSrcElts param to is..Mask static function in ShuffleVectorInst.

Need to add NumSrcElts param to is..Mask functions in
ShuffleVectorInstruction class for better mask analysis. Mask.size() not
always matches the sizes of the permuted vector(s). Allows to better
estimate the cost in SLP and fix uses of the functions in other cases.

Differential Revision: https://reviews.llvm.org/D158449