[VP] Fix VPintrinsic::getStaticVectorLength for vp.merge|select

VPIntrinsic::getStaticVectorLength infers the operational vector length
of a VPIntrinsic instance from a type that is used with the intrinsic.
The function used the mask operand before. Yet, vp.merge|select do not
have a mask operand (in the predicating sense that the other VP
intrinsics are using them - it is a selection mask for them). Fallback
to the return type to fix this.

Reviewed By: kaz7

Differential Revision: https://reviews.llvm.org/D121913