[Bug c++/85052] Implement support for clang's __builtin_convertvector

[husseydevin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85052

--- Comment #7 from Devin Hussey <husseydevin at gmail dot com> ---
Wait, silly me, this isn't about optimizations, this is about patterns.

It does the same thing it was doing for this code:

typedef unsigned u32x2 __attribute__((vector_size(8)));
typedef unsigned long long u64x2 __attribute__((vector_size(16)));

u64x2 cvt(u32x2 in)
{
    return (u64x2) { (unsigned long long)in[0], (unsigned long long)in[1] };
}