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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@333613 91177308-0d34-0410-b5e6-96231b3b80d8