[WebAssembly] v8x16.swizzle and rewrite BUILD_VECTOR lowering

Summary:
Adds the new v8x16.swizzle SIMD instruction as specified at
https://github.com/WebAssembly/simd/blob/master/proposals/simd/SIMD.md#swizzling-using-variable-indices.
In addition to adding swizzles as a candidate lowering in
LowerBUILD_VECTOR, also rewrites and simplifies the lowering to
minimize the number of replace_lanes necessary rather than trying to
minimize code size. This leads to more uses of v128.const instead of
splats, which is expected to increase performance.

The new code will be easier to tune once V8 implements all the vector
construction operations, and it will also be easier to add new
candidate instructions in the future if necessary.

Reviewers: aheejin, dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68527

llvm-svn: 374188