[WebAssembly] Improve macro hygiene in wasm_simd128.h

Summary:
The shuffle intrinsic macros did not parenthesize usages of their
constant parameters, which could lead to incorrect results due to
operator precedence issues. This patch fixes the problem by adding the
missing paretheses.

Reviewers: aheejin

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80968