[DAGCombiner] Account for getVectorIdxTy() when narrowing vector load

This patch makes ReplaceExtractVectorEltOfLoadWithNarrowedLoad convert
the element number from getVectorIdxTy() to PtrTy before doing pointer
arithmetic on it.  This is needed on z, where element numbers are i32
but pointers are i64.

Original patch by Richard Sandiford.

llvm-svn: 236530