[RISCV] Teach RISCVInsertVSETVLI to work without LiveIntervals (#94686)

Stacked on https://github.com/llvm/llvm-project/pull/94658.
    
We recently moved RISCVInsertVSETVLI from before vector register
allocation to after vector register allocation. When doing so, we added
an unconditional dependency on LiveIntervals - even at O0 where
LiveIntevals hadn't previously run. As reported in #93587, this was
apparently not safe to do.
    
This change makes LiveIntervals optional, and adjusts all the update
code to only run wen live intervals is present. The only real tricky
part of this change is the abstract state tracking in the dataflow. We
need to represent a "register w/unknown definition" state - but only
when we don't have LiveIntervals.
    
This adjust the abstract state definition so that the AVLIsReg state can
represent either a register + valno, or a register + unknown definition.
With LiveIntervals, we have an exact definition for each AVL use.
Without LiveIntervals, we treat the definition of a register AVL as
being unknown.
    
The key semantic change is that we now have a state in the lattice for
which something is known about the AVL value, but for which two
identical lattice elements do *not* necessarily represent the same AVL
value at runtime. Previously, the only case which could result in such
an unknown AVL was the fully unknown state (where VTYPE is also fully
unknown). This requires a small adjustment to hasSameAVL and lattice
state equality to draw this important distinction.
    
The net effect of this patch is that we remove the LiveIntervals
dependency at O0, and O0 code quality will regress for cases involving
register AVL values.
    
This patch is an alternative to
https://github.com/llvm/llvm-project/pull/93796 and
https://github.com/llvm/llvm-project/pull/94340. It is very directly
inspired by review conversation around them, and thus should be
considered coauthored by Luke.

0 files changed, 0 insertions, 0 deletions