[SVE][Analysis]Instruction costs for ops on scalable-vec

The following operations have no associated cost for them
when applied to scalable vectors, and as a consequence
can trigger a crash when a call is made to
AArch64TTIImpl::getCastInstrCost():
- fptrunc
- trunc
- fpext
- fpto(u,s)i

This patch adds costs for these operations and
relevant regression tests.

Differential Revision: https://reviews.llvm.org/D98934