[WebAssembly] Implement vector sext_inreg and tests with comparisons

Summary: Depends on D53251.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D53252

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344826 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed