[GlobalISel] Fix a bug in LegalizeRuleSet::clampMaxNumElements

Summary:
This function was causing a crash when `MaxElements == 1` because
it was trying to create a single element vector type.

Reviewers: dsanders, aemerson, aditya_nandakumar

Reviewed By: dsanders

Subscribers: rovka, kristof.beyls, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D53734

llvm-svn: 345875