Add a new VPInstruction::ReductionStartVector opcode to create the start
values for wide reductions. This more accurately models the start value
creation in VPlan and simplifies VPReductionPHIRecipe::execute. Down the
line it also allows removing VPReductionPHIRecipe::RdxDesc.

PR: https://github.com/llvm/llvm-project/pull/142290

Extend sinking logic to duplicate scalar steps recipe if it enables
sinking, that is if all users in a destination block require all lanes.

This should be the last step before removing legacy sinkScalarOperands.

PR: https://github.com/llvm/llvm-project/pull/136021

Considering that "or disjoint" is the canonical for certain add
operations, then I think we want to support such "add like" operations
when doing ADD+GEP->GEP+GEP rewrites to make things more consistent.

Problem was found when improving ValueTracking, which turned an ADD into
OR, and then suddenly optimizations got worse due to these rewrites no
longer triggering.

Follow-up to dfca6c0d3bf9d1a056 to extend isUnrolled handle any unrolled
VPlan, which means there's a single UF, but it will be > 1 if unrolling
took place.

After unrolling, there may be additional simplifications that can be
applied. One example is removing SCALAR-STEPS for the first part where
only the first lane is demanded.

This removes redundant adds of 0 from a large number of tests (~200),
many which I am still working on updating.

In preparation for removing redundant WideIV steps added in
https://github.com/llvm/llvm-project/pull/119284.

PR: https://github.com/llvm/llvm-project/pull/123655

Update and generalize materializeBroadcasts to also introduce explicit
broadcasts for VPValues defined in the Plans Entry block.

This fixes a crash when trying to insert the broadcasts generated by
VPTransformState::get after the generating instruction, which isn't
possible after invoke instructions.

Fixes https://github.com/llvm/llvm-project/issues/128838.

Avoid negations for normalized blends by reordering operands.

PR: https://github.com/llvm/llvm-project/pull/128375

Add a new VPInstruction::Broadcast opcode and use it to materialize
explicit broadcasts of live-ins. The initial patch only materlizes the
broadcasts if the vector preheader dominates all uses that need it.
Later patches will pick the best valid insert point, thus retiring
implicit hoisting of broadcasts from VPTransformsState::get().

PR: https://github.com/llvm/llvm-project/pull/124644

Follow-up as discussed when using VPInstruction::ResumePhi for all resume
values (#112147). This patch explicitly adds incoming values for each
predecessor in VPlan. This simplifies codegen and allows transformations
adjusting the predecessors of blocks with

NFC modulo incoming block order in phis.

Simplifies the code slightly and avoids redundant extracts/broadcasts if
the operand is live-in or already scalar.