[INSTCOMBINE] Transform reduction(shuffle V, poison, unique_mask) to reduction(V).

After SLP + LTO we may have have reduction(shuffle V, poison,
mask). This can be simplified to just reduction(V) if the mask is only
for single vector and just all elements from this vector are permuted,
  without reusing, replacing with undefs and/or other values, etc.

Differential Revision: https://reviews.llvm.org/D105053