Fix issue 17562 - tan returning -nan for inputs where abs(x) >= 2^63

The fptan instruction pushes a 1.0 onto the FPU register stack after a
successful operation, but when abs(input) >= 2^63 the C2 flag is set to
indicate that the input was out of bounds, and it doesn't push the 1.0.

Prior to this PR, the top value of the FPU stack was popped irrespective
of whether C2 was set, which in the case of an out-of-bounds input
caused the input to be removed from the stack and ultimately resulted in
an incorrect return value of -nan. This PR changes this behavior, only
popping after fptan when C2 was not set.

See: http://x86.renejeschke.de/html/file_module_x86_id_109.html

* Added unit tests for handling out-of-range inputs of fptan

std/math.d

@@ -768,10 +768,9 @@ real tan(real x) @trusted pure nothrow @nogc
         jc      trigerr                 ; // x is NAN, infinity, or empty
                                           // 387's can handle subnormals
 SC18:   fptan                           ;
-        fstp    ST(0)                   ; // dump X, which is always 1
         fstsw   AX                      ;
         sahf                            ;
-        jnp     Lret                    ; // C2 = 1 (x is out of range)
+        jnp     Clear1                  ; // C2 = 1 (x is out of range)
 
         // Do argument reduction to bring x into range
         fldpi                           ;
@@ -789,6 +788,10 @@ trigerr:
     }
     return real.nan;
 
+Clear1: asm pure nothrow @nogc{
+        fstp    ST(0)                   ; // dump X, which is always 1
+    }
+
 Lret: {}
     }
     else version(D_InlineAsm_X86_64)
@@ -815,10 +818,9 @@ Lret: {}
         jnz     trigerr                 ; // x is NAN, infinity, or empty
                                           // 387's can handle subnormals
 SC18:   fptan                           ;
-        fstp    ST(0)                   ; // dump X, which is always 1
         fstsw   AX                      ;
         test    AH,4                    ;
-        jz      Lret                    ; // C2 = 1 (x is out of range)
+        jz      Clear1                  ; // C2 = 1 (x is out of range)
 
         // Do argument reduction to bring x into range
         fldpi                           ;
@@ -837,6 +839,10 @@ trigerr:
     }
     return real.nan;
 
+Clear1: asm pure nothrow @nogc{
+        fstp    ST(0)                   ; // dump X, which is always 1
+    }
+
 Lret: {}
     }
     else
@@ -7174,6 +7180,12 @@ deprecated("Phobos1 math functions are deprecated, use isInfinity ") alias isinf
 
     real r = tan(-2.0L);
     assert(fabs(r - 2.18504f) < .00001);
+
+    // Verify correct behavior for large inputs
+    assert(!isNaN(tan(0x1p63)));
+    assert(!isNaN(tan(0x1p300L)));
+    assert(!isNaN(tan(-0x1p63)));
+    assert(!isNaN(tan(-0x1p300L)));
 }
 
 @safe pure nothrow unittest