Fix Neon builtin pointer argument checking for "sret" builtins.

The code for checking Neon builtin pointer argument types was assuming that
there would only be one pointer argument.  But, for vld2-4 builtins, the first
argument is a special sret pointer where the result will be stored.  So,
instead of scanning all the arguments to find a pointer, have TableGen figure
out the index of the pointer argument that needs checking.  That's better than
scanning all the arguments regardless.  <rdar://problem/10448804>

llvm-svn: 144834