[GlobalISel] Relax vector G_SELECT assertion.

For vector operands, the `select` instruction supports both vector and
non-vector conditions.  The MIR builder had an overly restrictive
assertion, that only accepted vector conditions for vector selects
(in effect implementing ISD::VSELECT).

Make it possible to express the full range of G_SELECTs.

llvm-svn: 297207