[X86] Implement __builtin_ia32_vec_ext_v2si correctly even though we only use...

[X86] Implement __builtin_ia32_vec_ext_v2si correctly even though we only use it with an index of 0.

This builtin takes an index as its second operand, but the codegen hardcodes an index of 0 and doesn't use the operand. The only use of the builtin in the header file passes 0 to the operand so this works for that usage. But its more correct to use the real operand.

llvm-svn: 334054