After switching the headers to implement the intrinsics using SSE2
(see https://reviews.llvm.org/D86855), these builtins are now unused.
Only 3 remain -- builtin_ia32_emms, still used by _mm_empty(), and
builtin_ia32_vec_{ext,set}_v4si, used by _mm_insert_pi16 and
_mm_extract_pi16, which lower to generic non-MMX IR.
Also update the clang/www/builtins.py with mappings for the
newly-removed builtins to their corresponding intrinsic functions.