[RISCV][GlobalISel] Add lowerCall for calling convention

[lewis-revill]

Summary

This patch implements minimal support for lowering function calls to callees with arguments and/or return values according to the RISC-V calling convention. Simple non-aggregate types are supported.

Feedback is very much appreciated.

[lewis-revill]

Rebase, update test structure. Correct missing defs by adding CallReturnHandler.

[Joe]

If you're supporting a pointer value return type, should there be a test for this?

Other than that, this looks fine to me - although should wait until somebody more experienced looks at it.

Side note: Would it be worth, in the future, to outline the process of finding In/OutputArgs and ArgInfos to something like a computeOutgoingArgs/computeIncomingArgs that fills a list of ISD::In/OutputArg and ArgInfos? Or would this just obfuscate things?

[lewis-revill]

Add test for pointer return type

[lewis-revill]

Bug fix - add target flags to call instruction.

[lewis-revill]

Rebased, and updated where appropriate to continue to build correctly.

[arsenm]

llvm/lib/Target/RISCV/RISCVCallLowering.cpp

311

Same comment on the other half of the call lowering patches

[lewis-revill]

Use ValueAssigners to avoid rewriting generic infrastructure and use determineAndHandleAssignments instead.

[arsenm]

llvm/lib/Target/RISCV/RISCVCallLowering.cpp

237

Not sure why you're bothering to filter these out. splitToValueTypes should handle near everything (I'm aware of some assertions on weird non-byte element sized vectors)