This allows the use a single wider operation with a restricted EVL
instead of having to split and cover via decreasing powers-of-two sizes.
On RISCV, this avoids the need for a bunch of vslidedown and vslideup
instructions to extract subvectors, and VL toggles to switch between the
various widths.
Note there is a potential downside of using vp nodes; we loose any
generic DAG combines which might have applied to the split form.
Currently, when using a VP match context with `sd_context_match`, only Opcode matching is possible (`m_Opc(Opcode)`).
This PR suggest a way to make patterns with Operands (eg `m_Node`, `m_Add`, ...) works with a VP context.
This PR blocks another PR https://github.com/llvm/llvm-project/pull/102877.
Co-authored-by: v01dxyz <v01dxyz@v01d.xyz>
This patch also pick the MatchContext framework from DAGCombiner to an
indiviual header file to make the framework be used from other files in
llvm/lib/CodeGen/SelectionDAG/.
