The @llvm.amdgcn.cs.chain intrinsic is essentially a call. The call
parameters are bundled up into 2 intrinsic arguments, one for those that
should go in the SGPRs (the 3rd intrinsic argument), and one for those
that should go in the VGPRs (the 4th intrinsic argument). Both will
often be some kind of aggregate.

Both instruction selection frameworks have some internal representation
for intrinsics (G_INTRINSIC[_WITH_SIDE_EFFECTS] for GlobalISel,
ISD::INTRINSIC_[VOID|WITH_CHAIN] for DAGISel), but we can't use those
because aggregates are dissolved very early on during ISel and we'd lose
the inreg information. Therefore, this patch shortcircuits both the
IRTranslator and SelectionDAGBuilder to lower this intrinsic as a call
from the very start. It tries to use the existing infrastructure as much
as possible, by calling into the code for lowering tail calls.

This has already gone through a few rounds of review in Phab:

Differential Revision: https://reviews.llvm.org/D153761

This avoids relying on G_EXTRACT on unusual types, and also properly
decomposes structs into multiple registers. This also preserves the
LLTs in the memory operands.

Or at least the sibling call cases which the DAG already handles.

This merges more AMDGPU ABI lowering code into the generic call
lowering. Start cleaning up by factoring away more of the pack/unpack
logic into the buildCopy{To|From}Parts functions. These could use more
improvement, and the SelectionDAG versions are significantly more
complex, and we'll eventually have to emulate all of those cases too.

This is mostly NFC, but does result in some minor instruction
reordering. It also removes some of the limitations with mismatched
sizes the old code had. However, similarly to the merge on the input,
this is forcing gfx6/gfx7 to use the gfx8+ ABI (which is what we
actually want, but SelectionDAG is stuck using the weird emergent
ABI).

This also changes the load/store size for stack passed EVTs for
AArch64, which makes it consistent with the DAG behavior.

I copied the nearly identical function from AArch64 into AMDGPU, so
fix this duplication.

Mips and X86 have their own more exotic versions which should be
removed. However replacing those is better left for a separate patch
since it requires other changes to avoid regressions.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D93813

There are various hacks working around limitations in
handleAssignments, and the logical split between different parts isn't
correct. Start separating the type legalization to satisfy going
through the DAG infrastructure from the code required to split into
register types. The type splitting should be moved to generic code.

If the return values can't be lowered to registers
SelectionDAG performs the sret demotion. This patch
contains the basic implementation for the same in
the GlobalISel pipeline.

Furthermore, targets should bring relevant changes
during lowerFormalArguments, lowerReturn and
lowerCall to make use of this feature.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D92953