[AArch64][GlobalISel] Fix bug in fewVectorElts action while legalizing oversize G_FPTRUNC vectors.

For <8 x s32> = fptrunc <8 x s64> the fewerElementsVector action tries to break
down the source vector into the final source vectors of <2 x s64> using unmerge.
This fixes a crash due to using the wrong number of elements for the breakdown
type.

Also add some legalizer tests for explicitly G_FPTRUNC which we didn't have.

Differential Revision: https://reviews.llvm.org/D87814