fixed mpfr_cospi() evaluation for MPFR version < 4.2

Author: wldfngrs

libc/utils/MPFRWrapper/MPFRUtils.cpp

@@ -255,19 +255,9 @@ class MPFRNumber {
     mpfr_cospi(result.value, value, mpfr_rounding);
     return result;
 #else
-    MPFRNumber value_frac(*this);
-    mpfr_frac(value_frac.value, value, MPFR_RNDN);
-
-    if (mpfr_cmp_si(value_frac.value, 0.0) == 0) {
-      mpz_t integer_part;
-      mpz_init(integer_part);
-      mpfr_get_z(integer_part, value, MPFR_RNDN);
-
-      if (mpz_tstbit(integer_part, 0)) {
-        mpfr_set_si(result.value, -1.0, MPFR_RNDN); // odd
-      } else {
-        mpfr_set_si(result.value, 1.0, MPFR_RNDN); // even
-      }
+    if (mpfr_integer_p(value)) {
+      auto d = mpfr_get_si(value, mpfr_rounding);
+      mpfr_set_si(result.value, (d & 1) ? -1 : 1, mpfr_rounding);
       return result;
     }
 
@@ -277,7 +267,7 @@ class MPFRNumber {
     mpfr_cos(result.value, value_pi.value, mpfr_rounding);
 
     return result;
-#endif
+//#endif
   }
 
   MPFRNumber erf() const {