[WebAssembly] Arbitrary BUILD_VECTOR and remove i64x2.mul

Summary:
This CL adds support for arbitrary BUILD_VECTORS, i.e. not splats and
not consts. This is the last feature needed to properly lower v2i64
multiplies without a i64x2.mul instruction (which is not in the spec),
so i64x2.mul is removed as well.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D51082

Remove unnecessary condition and fix whitespace

llvm-svn: 340472