Fix compiler warnings

- celt/modes.c:430:14: warning: cast from 'const unsigned char *' to
  'opus_int16 *' increases required alignment from 1 to 2 [-Wcast-align]
- 'C[0][1]' may be used uninitialized [-Wmaybe-uninitialized]
- Unused variable/parameter
- Value stored is never read
- MSVC warnings about "possible loss of data" due to type conversions
- MSVC warning C4146: unary minus operator applied to unsigned type
- silk/NLSF_del_dec_quant.c:137:20: warning: array subscript is above
  array bounds [-Warray-bounds] (gcc -O3 false positive)
- src/mlp_train.h:39:20: warning: function declaration isn't a prototype
  [-Wstrict-prototypes]
- Remove SMALL_FOOTPRINT code from SSE 4.1 FIR implementation, matching
  the C implementation.

The clang -Wcast-align warnings with SSE intrinsics are a known
clang issue: https://llvm.org/bugs/show_bug.cgi?id=20670

celt/fixed_debug.h

@@ -61,9 +61,11 @@ extern opus_int64 celt_mips;
 
 /** Add two 32-bit values, ignore any overflows */
 #define ADD32_ovflw(a,b) (celt_mips+=2,(opus_val32)((opus_uint32)(a)+(opus_uint32)(b)))
-/** Subtract two 32-bit values, ignore any overflows  */
+/** Subtract two 32-bit values, ignore any overflows */
 #define SUB32_ovflw(a,b) (celt_mips+=2,(opus_val32)((opus_uint32)(a)-(opus_uint32)(b)))
-#define NEG32_ovflw(a) (celt_mips+=2,(opus_val32)(-(opus_uint32)(a)))
+/* Avoid MSVC warning C4146: unary minus operator applied to unsigned type */
+/** Negate 32-bit value, ignore any overflows */
+#define NEG32_ovflw(a) (celt_mips+=2,(opus_val32)(0-(opus_uint32)(a)))
 
 static OPUS_INLINE short NEG16(int x)
 {