amd64 front end: add more spec rules:

amd64 front end: add more spec rules:

   S  after SHRQ
   Z  after SHLQ
   NZ after SHLQ
   Z  after SHLL
   S  after SHLL

The lack of at least one of these was observed to cause occasional false
positives in Memcheck.

Plus add commented-out cases so as to complete the set of 12 rules
{Z,NZ,S,NS} after {SHRQ,SHLQ,SHLL}.  The commented-out ones are commented
out because I so far didn't find any use cases for them.