[RISCV][GlobalISel] Add lowerFormalArguments for calling convention

[lewis-revill]

Summary

This patch adds an IncomingValueHandler and IncomingValueAssigner, and implements minimal support for lowering formal arguments according to the RISC-V calling convention. Simple non-aggregate integer and pointer types are supported.

[lewis-revill]

Rebase, update test structure.

[Joe]

llvm/lib/Target/RISCV/RISCVCallLowering.cpp

171

Perhaps add a test for this?

173

Floats don't work here. HandleAssignments tries to truncate it. Best to leave it unsupported for now.

[lewis-revill]

Address comments relating to supported types.

[lewis-revill]

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

[arsenm]

llvm/lib/Target/RISCV/RISCVCallLowering.cpp

201

Same as other patches, don't copy your own splitToValueTypes

[lewis-revill]

Introduce an IncomingValueAssigner to allow us to call determineAndHandleAssignments.

[arsenm]

llvm/test/CodeGen/RISCV/GlobalISel/irtranslator/args.ll

258

Probably should have some byval, sret, and cases where the argument list runs out of registers. Same for return values / hidden sret