[SelectionDAG][RISCV][SVE] Harden fixed offset version of ComputeValueVTs against scalable offsets.

Use getFixedValue instead of getKnownMinValue to convert TypeSize
to uint64_t. I believe this would have caught the bug fixed by
D157872.

To prevent false failures, I had to treat a scalable 0 as if it
is fixed value.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D158115