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[WebAssembly] Split and recombine multivalue calls for ISel

Authored by tlively on Dec 13 2019, 3:15 PM.

Description

[WebAssembly] Split and recombine multivalue calls for ISel

Summary:
Multivalue calls both take and return an arbitrary number of
arguments, but ISel only supports one or the other in a single
instruction. To get around this, calls are modeled as two pseudo
instructions during ISel. These pseudo instructions, CALL_PARAMS and
CALL_RESULTS, are recombined into a single CALL MachineInstr in a
custom emit hook.

RegStackification and the MC layer will additionally need to be made
aware of multivalue calls before the tests will produce correct
output.

Reviewers: aheejin, dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71496