[CodeGen] Convert double -> __fp16 in one step.

Fix the CodeGen so that for types bigger than float, instead of
converting to fp16 via the sequence "InTy -> float -> fp16", we
perform conversions in just one step.  This avoids the double
rounding which potentially changes results from a natural
IEEE-754 operation.

rdar://17594379, rdar://17468714
Differential Revision: http://reviews.llvm.org/D4602
Part of: http://reviews.llvm.org/D8367

llvm-svn: 232968