Fix corruption of result number in LegalizeVectorOps.cpp

When VectorLegalizer::LegalizeOp creates a new SDValue after iterating
over its arguments, we need to refer to the same result number of the
new node that the original value used.

Reviewed by: cameron.mcinally

Differential Revision: https://reviews.llvm.org/D49805

llvm-svn: 337939