improve arm_correlate_q7 for CM0 (#178)

Compilers GCC/CLANG unable to detect code similarities and merge __SSATs.
Let's help them emitting better code.

Co-authored-by: Christophe Favergeon <48906714+christophe0606@users.noreply.github.com>

Source/FilteringFunctions/arm_correlate_q7.c

@@ -921,15 +921,15 @@ void arm_correlate_q7(
  const q7_t *pIn2 = pSrcB + (srcBLen - 1U); /* InputB pointer */
  q31_t sum; /* Accumulator */
  uint32_t i = 0U, j; /* Loop counters */
- uint32_t inv = 0U; /* Reverse order flag */
+ int32_t inc = 1;  /* Destination address modifier */
  uint32_t tot = 0U; /* Length */
 
  /* The algorithm implementation is based on the lengths of the inputs. */
  /* srcB is always made to slide across srcA. */
  /* So srcBLen is always considered as shorter or equal to srcALen */
  /* But CORR(x, y) is reverse of CORR(y, x) */
  /* So, when srcBLen > srcALen, output pointer is made to point to the end of the output buffer */
- /* and a varaible, inv is set to 1 */
+ /* and a varaible, inc is set to -1 */
  /* If lengths are not equal then zero pad has to be done to make the two
  * inputs of same length. But to improve the performance, we include zeroes
  * in the output instead of zero padding either of the the inputs*/
@@ -968,8 +968,8 @@ void arm_correlate_q7(
  srcALen = srcBLen;
  srcBLen = j;
 
- /* Setting the reverse flag */
- inv = 1;
+ /* Filling destination in reverse order */
+ inc = -1;
  }
 
  /* Loop to calculate convolution for output length number of times */
@@ -990,10 +990,8 @@ void arm_correlate_q7(
  }
 
  /* Store the output in the destination buffer */
- if (inv == 1)
- *pDst-- = (q7_t) __SSAT((sum >> 7U), 8U);
- else
- *pDst++ = (q7_t) __SSAT((sum >> 7U), 8U);
+ *pDst = (q7_t) __SSAT((sum >> 7U), 8U);
+ pDst += inc;
  }
 
 #endif /* #if !defined(ARM_MATH_CM0_FAMILY) */