A previous patch fixed vldrd/vstrd by dividing the immediate offset of
the instruction by 4 before encoding. This does the same for
vldrs/vstrs. It fixes the remaining problems with compiling spec2k
using -filetype=iasm.

It also fixes a minor bug in the divsion by 4, in the case that the
immediate value is negative.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4334
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1617993005 .