[X86] Remove __extension__ from macro intrinsics when its not needed.

I think this is a holdover from when we used to declare variables inside the macros. And then its been copy and pasted forward for years every time a new macro intrinsic gets added.

Interestingly this caused some tests for IRGen to be slightly more optimized. We now return a zeroinitializer directly instead of going through a store+load.

It also removed a bogus error message on another test.

llvm-svn: 333613