This patch adds a new VPInstruction::HeaderMask opcode to model the
abstract header-mask used for tail-folding. It will be lowered depending
on target preference (either using active-lane-mask,
explicit-vector-length or a wide compare of the canonical IV and the
backedge taken count)

Similarly to llvm#82270, it would
be good to clarify/agree on the terminology w.r.t. to recipes/opcodes
that cannot be code-gen'd directly (i.e. require further gradual lowering).

NOTE: some tests are failing or needed updating, due to widened IVs
being replaced by scalar-steps, as their only use was the earlier wide
compare. This could be fixed by either adding a suitable wide canonical
IV as operand to the header-mask recipe and exactly preserve the
original behavior. Alternatively we could keep the current behavior of
the patch and update the tests. Or introduce a wide induction PHI
instead of VPWidenCanonicalIVReicpe; currently we *only* use a wide IV
for VPWidenCanonicalIVRecipe, if there was a suitable IV in the original
loop, *even* if the mask compare is the *only* wide use. Either never or
always using a wide PHI would be more consistent (or eventually make a
more informed cost-based decision).