[DAGCombiner] Fix ReplaceExtractVectorEltOfLoadWithNarrowedLoad for BE

For little-endian, the function would convert (extract_vector_elt (load X), Y)
to X + Y*sizeof(elt).  For big-endian it would instead use
X + sizeof(vec) - Y*sizeof(elt).  The big-endian case wasn't right since
vector index order always follows memory/array order, even for big-endian.
(Note that the current handling has to be wrong for Y==0 since it would
access beyond the end of the vector.)

Original patch by Richard Sandiford.

llvm-svn: 236529