Fix constant folding through bitcasts by no longer relying on undefined...

Fix constant folding through bitcasts by no longer relying on undefined behaviour (converting NaN values between float and double).

SelectionDAG::getConstantFP(double Val, EVT VT, bool isTarget);
should not be used when Val is not a simple constant (as the comment in
SelectionDAG.h indicates). This patch avoids using this function
when folding an unknown constant through a bitcast, where it cannot be
guaranteed that Val will be a simple constant.

llvm-svn: 163703