[LoopVectorize] Fix assertion failure in fixReduction when tail-folding

When compiling the attached new test in scalable-reductions-tf.ll we
were hitting this assertion in fixReduction:

  Assertion `isa<PHINode>(U) && "Reduction exit must feed Phi's or select"

The loop contains a reduction and an intermediate store of the reduction
value. When vectorising with tail-folding the contains of 'U' in the
assertion above happened to be a scatter_store. It turns out that we
were still creating a widen recipe for the invariant store, despite
knowing that we can actually sink it. The simplest fix is to change
buildVPlanWithVPRecipes so that we look for invariant stores before
attempting to widen it.

Differential Revision: https://reviews.llvm.org/D126295