Add constrained fptrunc and fpext intrinsics.

The new fptrunc and fpext intrinsics are constrained versions of the
regular fptrunc and fpext instructions.

Reviewed by:	Andrew Kaylor, Craig Topper, Cameron McInally, Conner Abbot
Approved by:	Craig Topper
Differential Revision: https://reviews.llvm.org/D55897

llvm-svn: 360581