APFloat: Make sure that we get a well-formed x87 NaN when converting from a smaller type.

Fixes PR15054.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173459 91177308-0d34-0410-b5e6-96231b3b80d8
Rename variable to be more comprehensible and follow naming convention


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173460 91177308-0d34-0410-b5e6-96231b3b80d8