[SVE] Suppress vselect warning from incorrect interface call

The VSelectCombine handler within AArch64ISelLowering,
uses an interface call which only expects fixed vectors.
This generates a warning when the call is made on a
scalable vector. This warning has been suppressed with this change,
by using the ElementCount interface, which supports both fixed and scalable vectors.
I have also added a regression test which recreates the warning.

Differential Revision: https://reviews.llvm.org/D98249