[SVE] Remove calls to VectorType::getNumElements from WebASM

Summary:
The getNumElements in base VectorType is being deprecated.

See: http://lists.llvm.org/pipermail/llvm-dev/2020-March/139811.html

Reviewers: efriedma, tlively, fpetrogalli, c-rhodes, dschuff

Reviewed By: tlively, dschuff

Subscribers: dschuff, sbc100, tschuett, jgravelle-google, hiraditya, aheejin, rkruppe, psnobl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82217