[NFC][WebAssembly] Add tests for alignment on new SIMD loads

Summary:
The natural alignments for extending and splatting loads had not
previously been tested. It is good to have them tested because they
are non-obvious details in the SIMD spec proposal.

Reviewers: aheejin

Subscribers: dschuff, sbc100, jgravelle-google, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81303