Add target implementations for the X86 builtins:

  __builtin_ia32_vec_init_v8qi
  __builtin_ia32_vec_init_v4hi
  __builtin_ia32_vec_init_v2si

They are lowered to bitcasts. (These are all ready tested by the gcc testsuite.)
<rdar://problem/8529957>

llvm-svn: 116147