[flang] Fix folding of NEAREST(TINY(1.),-1.) (llvm#76590)

The code to fold NEAREST would return a value that's too large when
transitioning from a normal number to a subnormal.

Fixes llvm-test-suite/Fortran/gfortran/regression/nearest_1.f90.

Author: klausler

flang/lib/Evaluate/real.cpp

@@ -350,7 +350,7 @@ ValueWithRealFlags<Real<W, P>> Real<W, P>::NEAREST(bool upward) const {
         isNegative = !isNegative;
       } else {
         auto sub1{fraction.SubtractSigned(one)};
-        if (sub1.overflow) {
+        if (sub1.overflow && expo > 1) {
           nearest = Fraction{0}.NOT();
           --expo;
         } else {

flang/test/Evaluate/fold-nearest.f90

@@ -26,6 +26,8 @@ module m1
   logical, parameter :: test_14 = nearest(0., negZero) == -minSubnormal
   !WARN: warning: NEAREST: S argument is zero
   logical, parameter :: test_15 = nearest(negZero, 0.) == minSubnormal
+  logical, parameter :: test_16 = nearest(tiny(1.),-1.) == 1.1754942E-38
+  logical, parameter :: test_17 = nearest(tiny(1.),1.) == 1.1754945E-38
 end module
 
 module m2