lavc/aacenc_utils: replace sqrtf(Q*sqrtf(Q)) by precomputed value

It makes no sense whatsoever to do this at each function call; we
already have a table for this.

Yields a 2x improvement in find_min_book (x86-64, Haswell+GCC):
ffmpeg -i sin.flac -acodec aac -y sin.aac
find_min_book
old
    605 decicycles in find_min_book, 8388453 runs,    155 skips.9x
    606 decicycles in find_min_book,16776912 runs,    304 skips.9x
    607 decicycles in find_min_book,33553819 runs,    613 skips.2x
    607 decicycles in find_min_book,67107668 runs,   1196 skips.3x
    607 decicycles in find_min_book,134215360 runs,   2368 skips3x

new
    359 decicycles in find_min_book, 8388552 runs,     56 skips.3x
    360 decicycles in find_min_book,16777112 runs,    104 skips.1x
    361 decicycles in find_min_book,33554218 runs,    214 skips.4x
    361 decicycles in find_min_book,67108381 runs,    483 skips.5x
    361 decicycles in find_min_book,134216725 runs,   1003 skips5x

and more importantly a non-negligible speedup (~ 8%) to overall AAC encoding:
old:
ffmpeg -i sin.flac -acodec aac -strict -2 -y sin_new.aac  6.82s user 0.03s system 104% cpu 6.565 total
new:
ffmpeg -i sin.flac -acodec aac -strict -2 -y sin_old.aac  6.24s user 0.03s system 104% cpu 5.993 total

This also improves accuracy of the expression by ~ 2 ulp in some cases.
Reviewed-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Reviewed-by: Rostislav Pehlivanov <atomnuker@gmail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanag@gmail.com>

libavcodec/aacenc_utils.h

@@ -90,8 +90,7 @@ static inline float find_max_val(int group_len, int swb_size, const float *scale
 
 static inline int find_min_book(float maxval, int sf)
 {
-    float Q = ff_aac_pow2sf_tab[POW_SF2_ZERO - sf + SCALE_ONE_POS - SCALE_DIV_512];
-    float Q34 = sqrtf(Q * sqrtf(Q));
+    float Q34 = ff_aac_pow34sf_tab[POW_SF2_ZERO - sf + SCALE_ONE_POS - SCALE_DIV_512];
     int qmaxval, cb;
     qmaxval = maxval * Q34 + C_QUANT;
     if (qmaxval >= (FF_ARRAY_ELEMS(aac_maxval_cb)))