[WebAssembly][NFC] Remove duplicate SIMD instructions and predicates

Summary:
Instead of having different v128.load and v128.store instructions for
each MVT, just have one of each that is reused in all the
patterns. Also removes the HasSIMD128 predicate where accompanied by
HasUnimplementedSIMD128, since the latter implies the former.

Reviewers: aheejin, dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67930

llvm-svn: 372792