[WebAssembly] Update comments for non-splat pow2 vector test case

Summary:
After rL335727, (sdiv X, 1) is treated as a special case, so we can
safely transform 'sdiv's in non-splat pow vectors into 'shr's even when
some of its entries are '1'. The test expectations have been already
fixed in rL335771, but the comments were out of date.

Also changed the filename from `vector_sdiv.ll` to `vector-sdiv.ll` to
be consistent with other test file names.

Reviewers: RKSimon

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

Differential Revision: https://reviews.llvm.org/D48692

llvm-svn: 336018