[NFC] Bail out for scalable vectors before calling getNumElements

Summary:
Move the bail out logic to before constructing the Result and Lane
vectors. This is both potentially faster, and avoids calling
getNumElements on a potentially scalable vector

Reviewers: efriedma, sunfish, chandlerc, c-rhodes, fpetrogalli

Reviewed By: fpetrogalli

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81619