For SSE2 and SSE3, there are four states that the two flags
involved (AV_CPU_FLAG_SSE[23] and AV_CPU_FLAG_SSE[23]SLOW) can convey.
When ordered from worst to best they are:
1. both flags unset (SSE[23] unavailable)
2. the slow flag set, the ordinary flag unset (this is designed
for cases where SSE2 is available, but so slow that MMX(EXT)/SSE
code is usually faster)
3. both flags set (SSE2 is available, but there might be scenarios
where MMX(EXT)/SSE code is faster)
4. the ordinary flag set, the slow flag unset (this is the normal case)

The ordinary macros for checking cpuflags return true
in the latter two cases; the fast macros only return true for
the latter case. Yet the macros to check for slow currently
only return true in case three.

This seems unintended. In fact, the only uses of the slow macros
are all of the form
if (EXTERNAL_SSE2(cpu_flags) || EXTERNAL_SSE2_SLOW(cpu_flags))
where the check for EXTERNAL_SSE2_SLOW is completely redundant.
Even more importantly, it is not what was intended. Before
6369ba3c, the checks passed
in cases 2 to 4. Said commit changed this to something that
only passes for the third case. Commits
7fb758cd and
c1913064 restored the old behaviour,
yet merging 4efab893 (in commit
ac774cfa) broke this again
by changing it to what it is now.*

This commit changes the macros to make the slow macros check
whether a specific instruction is supported, even if slow.
This restores the intended meaning to all uses of the SLOW macros
and is generally more natural.

*: Libav only checks for EXTERNAL_SSE2_SLOW, i.e. for the third case
only.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>