[X86] Bale out of X86FastISel::X86SelectCmp for vectors.

None of the code in this function was written to handle
vectors.  Most of the cases already fail for vectors for one
reason or another. The exception is an optimization that
detects identical operands. This can be triggered by vectors,
but the code always creates a 0 or 1 constants in a scalar
register which is incorrect for vectors.

Fixes PR49706.