[SLP] Fix for PR32086: Count InsertElementInstr of the same elements as shuffle.

Summary:
If the same value is going to be vectorized several times in the same
tree entry, this entry is considered to be a gather entry and cost of
this gather is counter as cost of InsertElementInstrs for each gathered
value. But we can consider these elements as ShuffleInstr with
SK_PermuteSingle shuffle kind.

Reviewers: spatel, RKSimon, mkuper, hfinkel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38697

llvm-svn: 323246