Revert "[APFloat] Removed APFloat constructor which initialized to either...

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.

llvm-svn: 185099