DAGCombiner: StoreMerging: Fix bad index calculating when adjusting mismatching vector types

This fixes a case of bad index calculation when merging mismatching
vector types. This changes the existing code to just use the existing
extract_{subvector|element} and a bitcast (instead of bitcast first and
then newly created extract_xxx) so we don't need to adjust any indices
in the first place.

rdar://44584718

Differential Revision: https://reviews.llvm.org/D52681

llvm-svn: 343493