[SLP] Fix vectorization for tree with trunc to minimum required bit width.

Summary:
If the vectorized tree has truncate to minimum required bit width and
the vector type of the cast operation after the truncation is the same
as the vector type of the cast operands, count cost of the vector cast
operation as 0, because this cast will be later removed.
Also, if the vectorization tree root operations are integer cast operations, do not consider them as candidates for truncation. It will just create extra number of the same vector/scalar operations, which will be removed by instcombiner.

Reviewers: RKSimon, spatel, mkuper, hfinkel, mssimpso

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41948

llvm-svn: 322946