Revert "[APFloat] Removed APFloat constructor which initialized to either zero/NaN but allowed you to arbitrarily set the category of the float."

This reverts commit r185095. This is causing a FileCheck failure on
the 3dnow intrinsics on at least the mips/ppc bots but not on the x86
bots.

Reverting while I figure out what is going on.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185099 91177308-0d34-0410-b5e6-96231b3b80d8