[X86] Remove __builtin_ia32_padd/psub saturated intrinsics and use generic...

[X86] Remove __builtin_ia32_padd/psub saturated intrinsics and use generic __builtin_elementwise_add/sub_sat

D117898 added the generic __builtin_elementwise_add_sat and __builtin_elementwise_sub_sat with the same integer behaviour as the SSE/AVX instructions

This patch removes the __builtin_ia32_padd/psub saturated intrinsics and just uses the generics - the existing tests see no changes:

__m256i test_mm256_adds_epi8(__m256i a, __m256i b) {
  // CHECK-LABEL: test_mm256_adds_epi8
  // CHECK: call <32 x i8> @llvm.sadd.sat.v32i8(<32 x i8> %{{.*}}, <32 x i8> %{{.*}})
  return _mm256_adds_epi8(a, b);
}