[AArch64] Don't look at type size for scalable types in isExtFreeImpl

This fixes one of those 'Request for a fixed element count on a scalable
object' errors in the AArch64 isExtFreeImpl method, where the uses of a sext
are checked to see if the instruction can be considered free.
https://godbolt.org/z/debYP9c4G

Differential Revision: https://reviews.llvm.org/D152930