Elementwise builtins are not constexpr

[RKSimon]

https://simd.godbolt.org/z/E4KcoTfdP

typedef int __v4si __attribute__((__vector_size__(16)));
constexpr __v4si minmax(__v4si x, __v4si y) {
    return __builtin_elementwise_max(x, y);
}

clang -g0 -O3

<source>:3:18: error: constexpr function never produces a constant expression [-Winvalid-constexpr]
constexpr __v4si minmax(__v4si x, __v4si y) {
                 ^
<source>:4:12: note: subexpression not valid in a constant expression
    return __builtin_elementwise_max(x, y);

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[RKSimon]

Duplicate of #51787