DAG combine incorrectly optimize (i32 vextract (v4i16 load $addr), c) to

(i16 load $addr+c*sizeof(i16)) and replace uses of (i32 vextract) with the
i16 load. It should issue an extload instead: (i32 extload $addr+c*sizeof(i16)).

rdar://11035895

llvm-svn: 152675