Translate NEON vaddl, vaddw, vsubl, and vsubw builtins to llvm add/sub

with zext/sext operations, instead of to llvm intrinsics.  (We can also
get rid of the clang builtins and handle these entirely in the arm_neon.h
header if there is a way to express vector sext/zext in C.)

llvm-svn: 112413