[COST]Fix PR35865: Fix cost model evaluation for shuffle on X86.

Summary:
If the vector type is transformed to non-vector single type, the compile
may crash trying to get vector information about non-vector type.

Reviewers: RKSimon, spatel, mkuper, hfinkel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41862

llvm-svn: 322106