[AArch64][GlobalISel] Use custom legalization for G_TRUNC for v8i8 vectors.

Truncating to v8i8 is a case where we want to split the source but also generate
intermediate truncates to reduce the size of the source vector before truncating
down to v8i8. This implements the same strategy as what SelectionDAG does, but
I'm not certain where if anywhere in generic code it should live.

Use it for legalization of v8s8 = G_ICMP v8s32.

Differential Revision: https://reviews.llvm.org/D88191