As discussed in https://github.com/llvm/llvm-project/pull/140405, remove
custom printing for resume-phis and update tests.

By using SINGLE-SCALAR when printing, it is clear in the debug output
that those VPInstructions only produce a single scalar.

Split off in preparation for
https://github.com/llvm/llvm-project/pull/140623.

PR: https://github.com/llvm/llvm-project/pull/141428

Update initial construction to connect the Plan's entry to the scalar
preheader during initial construction. This moves a small part of the
 skeleton creation out of ILV and will also enable replacing
 VPInstruction::ResumePhi with regular VPPhi recipes.

Resume phis need 2 incoming values to start with, the second being the
bypass value from the scalar ph (and used to replicate the incoming
value for other bypass blocks). Adding the extra edge ensures we
incoming values for resume phis match the incoming blocks.

PR: https://github.com/llvm/llvm-project/pull/140132

ExtractFromEnd only has 2 uses, extracting the last and penultimate
elements. Replace it with 2 separate opcodes, removing the need to
materialize and handle a constant argument.

PR: https://github.com/llvm/llvm-project/pull/137030

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

Similarly to other recipes, update VPScalarIVStepsRecipe to also take
the runtime VF as argument. This removes some unnecessary runtime VF
computations for scalable vectors. It will also allow dropping the
UF == 1 restriction for narrowing interleave groups required in
577631f0a528.

Splits off reduction printing tests, to limit growth and add test case
for printing find-last-IV (https://github.com/llvm/llvm-project/pull/132689)

VPReductionRecipes take a RecurrenceDescriptor, but only use the
RecurKind and FastMathFlags in it when executing. This patch makes the
recipe more lightweight by stripping it to only take the latter two.

The motiviation for this is to simplify an upcoming patch to support
in-loop AnyOf reductions. For an in-loop AnyOf reduction we want to
create an Or reduction, and by using RecurKind we can create an
arbitrary reduction without needing a full RecurrenceDescriptor.

Currently fast() won't return true if all flags are set via setXXX,
which is surprising. Update setters to set all bits if needed to make
sure isFast() consistently returns the expected result.

PR: https://github.com/llvm/llvm-project/pull/131321

Change the inheritance of class VPWidenSelectRecipe to class
VPRecipeWithIRFlags, which allows recipe of the select to pass the
fastmath flags.The patch of #119847 will add the fastmath flag to for
recipe