[SVE][AArch64] Fix TypeSize warning in GEP cost analysis

The warning would fire when calling getGEPCost for analyzing the cost of
a GEP instruction. This would result in the use of the now deprecated
implicit cast of TypeSize to uint64_t through the overloaded operator.

This patch fixes the issue by using getKnownMinSize instead of the
implicit cast. This is possible because the code is already
scalable-vector aware. The semantic behaviour of the code is unchanged
by this patch.

Reviewed By: sdesmalen, fpetrogalli

Differential Revision: https://reviews.llvm.org/D89872