Match one function, update mips_to_c of 2 others (#164)

There's a function that appears to use continued fractions
 to approximate the value of tanh(x/2), which is super weird.

Signed-off-by: Taggerung <tyler.taggerung@gmail.com>

src/code_80091750.h

@@ -47,6 +47,7 @@ typedef struct {
 /* Function Prototypes */
 
 f64 exponent_by_squaring(f64, s32);
+f64 func_80091A6C(f64, s32);
 void swap_values(s32*, s32*);
 void func_80092148();
 void func_800921B4();