llvm-stress: stop abusing ConstantFP::get()

ConstantFP::get(Type*, double) is unreliably host-specific:
it can't handle a type like PPC128 on an x86 host. It even
has a comment to that effect: "This should only be used for
simple constant values like 2.0/1.0 etc, that are
known-valid both as host double and as the target format."

Instead, use APFloat. While we're at it, randomize the floating
point value more thoroughly; it was previously limited
to the range 0 to 2**19 - 1.

PR12451.

llvm-svn: 154446