block_connected: re-use previous GetTimeMicros

Shave off an extra 100 or so instructions from the
validation:block_connected tracepoint by reusing a nearby
GetTimeMicros(). This brings the tracepoint down to 54 instructions.
Still high, but much better than the previous ~154 and 8000 instructions
which it was originally.

Signed-off-by: William Casarin <jb55@jb55.com>

src/validation.cpp

@@ -2166,7 +2166,7 @@ bool CChainState::ConnectBlock(const CBlock& block, BlockValidationState& state,
         block.vtx.size(),
         nInputs,
         nSigOpsCost,
-        GetTimeMicros() - nTimeStart // in microseconds (µs)
+        nTime5 - nTimeStart // in microseconds (µs)
     );
 
     return true;