[flang] Fix float-number representation bug

The float number is represented as (-1)^s * 1.f * 2^(-127) for 32-bit,
where s is the signed flag, f is the mantissa. When the exponent bits
are all zeros, the float number is represented as (-1)^s * 0.f *2^(-126)
for 32-bit, in which case, the intPart is '0'.

Reviewed By: Jean Perier

https://reviews.llvm.org/D123673