[SVE][ISel] Ensure explicit gather/scatter offset extension isn't lost.

getGatherScatterIndexIsExtended currently looks through all
SIGN_EXTEND_INREG operations regardless of their input type.  This
patch restricts the code to only look through i32->i64 extensions,
which are the ones supported implicitly by SVE addressing modes.

Differential Revision: https://reviews.llvm.org/D123318