[WebAssembly] Retain shuffle types during custom lowering

Summary:
Changing the node type in lowering was violating assumptions made in
the DAG combiner, so don't change the node type any more. This fixes
one of the issues reported in bug 39275.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D53537

llvm-svn: 345221