To deduce whether the optimization is legal we need to compare the target
features between caller and callee versions. The criteria for bypassing
the resolver are the following:

 * If the callee's feature set is a subset of the caller's feature set,
   then the callee is a candidate for direct call.

 * Among such candidates the one of highest priority is the best match
   and it shall be picked, unless there is a version of the callee with
   higher priority than the best match which cannot be picked from a
   higher priority caller (directly or through the resolver).

 * For every higher priority callee version than the best match, there
   is a higher priority caller version whose feature set availability
   is implied by the callee's feature set.

Example:

Callers and Callees are ordered in decreasing priority.
The arrows indicate successful call redirections.

  Caller        Callee      Explanation
=========================================================================
mops+sve2 --+--> mops       all the callee versions are subsets of the
            |               caller but mops has the highest priority
            |
     mops --+    sve2       between mops and default callees, mops wins

      sve        sve        between sve and default callees, sve wins
                            but sve2 does not have a high priority caller

  default -----> default    sve (callee) implies sve (caller),
                            sve2(callee) implies sve (caller),
                            mops(callee) implies mops(caller)

* clang format
* remove leftover target hook hasFMV after rebase
* remove filter in regression test after rebase