[WebAssembly] Optimize splats of bitcasted vectors

Summary:
This new custom DAG combine fixes a codegen issue with the
wasm_simd128.h intrinsics. Clang lowers the

  return (v128_t)(__f32x4){__a, __a, __a, __a};

body of f32x4_splat to a splat shuffle of a bitcasted vector, as seen
in the new simd-shuffle-bitcast.ll test. The bitcast interfered with
the target-independent DAG combine that combines splat shuffles into
BUILD_VECTOR nodes, so this patch introduces a new custom DAG combine
to hoist the bitcast out of the shuffle, allowing the
target-independent combine to work as intended.

Reviewers: aheejin, dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80021