Move getStrideFromPointer and friends from LoopVectorize to VectorUtils

The following functions are moved from the LoopVectorizer to VectorUtils:

  - getGEPInductionOperand
  - stripGetElementPtr
  - getUniqueCastUse
  - getStrideFromPointer

These used to be static functions in LoopVectorize, but will also be used by
the upcoming loop versioning LICM transformation.

Patch by Ashutosh Nema!

llvm-svn: 241980