convert fptosi(sitofp x) -> x if the fp value has enough bits in its mantissa

convert fptosi(sitofp x) -> x  if the fp value has enough bits in its mantissa
to accurately represent the integer.  This triggers 9 times in 471.omnetpp,
though 8 of those seem to be inlined from the same place.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51271 91177308-0d34-0410-b5e6-96231b3b80d8