[X86] Remove `__builtin_ia32_pmax/min` intrinsics and use generic `__builtin_elementwise_max/min`

D111985 added the generic `__builtin_elementwise_max` and `__builtin_elementwise_min` intrinsics with the same integer behaviour as the SSE/AVX instructions

This patch removes the `__builtin_ia32_pmax/min` intrinsics and just uses `__builtin_elementwise_max/min` - the existing tests see no changes:
```
__m256i test_mm256_max_epu32(__m256i a, __m256i b) {
  // CHECK-LABEL: test_mm256_max_epu32
  // CHECK: call <8 x i32> @llvm.umax.v8i32(<8 x i32> %{{.*}}, <8 x i32> %{{.*}})
  return _mm256_max_epu32(a, b);
}
```
This requires us to add a `__v64qs` explicitly signed char vector type (we already have `__v16qs` and `__v32qs`).

Sibling patch to D117791

Differential Revision: https://reviews.llvm.org/D117798