Revert "[WebAssembly] Split and recombine multivalue calls for ISel"

Summary:
This reverts commit 28857d14. This
commit worked toward a solution that did not turn out to be feasible
because MachineInstrs cannot contain an arbitrary number of defs.

Reviewers: aheejin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73927