[LoopVectorize] Prevent multiple Phis being generated with in-order reductions

When using the -enable-strict-reductions flag where UF>1 we generate multiple
Phi nodes, though only one of these is used as an input to the vector.reduce.fadd
intrinsics. The unused Phi nodes are removed later by instcombine.

This patch changes widenPHIInstruction/fixReduction to only generate
one Phi, and adds an additional test for unrolling to strict-fadd.ll

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D100570