Add 3dNOW intrinsic header to x86intrin.h, conditioned on __3dNOW__ to

match the behavior of GCC. Also add a test for these intrinsics, which
apparently have *zero* tests. =[ Not surprisingly, Clang crashed when
compiling these.

Fix the bug in CodeGen where we failed to bitcast the argument type to
x86mmx prior to calling the LLVM intrinsic. This fixes an assert on the
new 3dnow-builtins.c test.

This is one issue impacting the efforts to get Clang to emulate the
Microsoft intrinsics headers -- 3dnow intrinsics are implictitly made
available there.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@150948 91177308-0d34-0410-b5e6-96231b3b80d8