Use ConstantFoldFP for folding all unary floating-point operations which may

Use ConstantFoldFP for folding all unary floating-point operations which may
have an error, and refector out the code for binary operators into
ConstantFoldBinaryFP and use it for all binary floating-point operations
which may have an error. These functions still rely exclusively on errno
to detect errors though.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@39923 91177308-0d34-0410-b5e6-96231b3b80d8