[WebAssembly] Add immarg attribute to intrinsics

Summary:
After r355981, intrinsic arguments that are immediate values should be
marked as `ImmArg`.

Reviewers: dschuff, tlively

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59447

llvm-svn: 356437