opusdsp: adjust and optimize C function to match assembly

The C and asm versions behaved differently _outside_ of the codec.

The C version returned pre-multiplied 'state' for the next execution
to use right away, while the assembly version outputted non-multiplied
'state' for the next execution to multiply to save instructions.
Since the initial state when initialized or seeking is always 0,
and since C and asm versions were never mixed, there was no issue.

However, comparing outputs directly in checkasm doesn't work without
dividing the initial state by CELT_EMPH_COEFF and multiplying the
returned state by CELT_EMPH_COEFF for the assembly function.

Since its actually faster to do this in C as well, copy the behavior the
asm versions use. As a reminder, the initial state 0 is divided by
CELT_EMPH_COEFF on seek and init (just in case in the future this is
changed, its technically more correct to init with CELT_EMPH_COEFF than 0,
however when seeking this will result in more audiable pops, unlike with 0
where the output gets in sync over a few samples).

libavcodec/opus_celt.c

@@ -507,7 +507,11 @@ void ff_celt_flush(CeltFrame *f)
         memset(block->pf_gains_old, 0, sizeof(block->pf_gains_old));
         memset(block->pf_gains_new, 0, sizeof(block->pf_gains_new));
 
-        block->emph_coeff = 0.0;
+        /* libopus uses CELT_EMPH_COEFF on init, but 0 is better since there's
+         * a lesser discontinuity when seeking.
+         * The deemphasis functions differ from libopus in that they require
+         * an initial state divided by the coefficient. */
+        block->emph_coeff = 0.0f / CELT_EMPH_COEFF;
     }
     f->seed = 0;
 

libavcodec/opusdsp.c

@@ -43,15 +43,10 @@ static void postfilter_c(float *data, int period, float *gains, int len)
 
 static float deemphasis_c(float *y, float *x, float coeff, int len)
 {
-    float state = coeff;
+    for (int i = 0; i < len; i++)
+        coeff = y[i] = x[i] + coeff*CELT_EMPH_COEFF;
 
-    for (int i = 0; i < len; i++) {
-        const float tmp = x[i] + state;
-        state = tmp * CELT_EMPH_COEFF;
-        y[i] = tmp;
-    }
-
-    return state;
+    return coeff;
 }
 
 av_cold void ff_opus_dsp_init(OpusDSP *ctx)