[DSE] Call isRemovable() after getLocForWriteEx() (NFCI)

The only non-trivial change here is that the isReadClobber()
check for redundant stores is now on the DefLoc, not the
UpperLoc. This is semantically the right location to use, though
in practice it makes no difference (the locations are either the
same, or the def inst does not read).