ISel: Make VSELECT selection terminate in cases where the condition type has to
be split and the result type widened.

When the condition of a vselect has to be split it makes no sense widening the
vselect and thereby widening the condition. We end up in an endless loop of
widening (vselect result type) and splitting (condition mask type) doing this.
Instead, split both the condition and the vselect and widen the result.

I ran this over the test suite with i686 and mattr=+sse and saw no regressions.

Fixes PR18036.