[WebAssembly] Codegen for v128.load{32,64}_zero

Replace the experimental clang builtins and LLVM intrinsics for these
instructions with normal instruction selection patterns. The wasm_simd128.h
intrinsics header was already using portable code for the corresponding
intrinsics, so now it produces the correct instructions.

Differential Revision: https://reviews.llvm.org/D106400