[RISCV] Add/remove/simplify tests in xqcisls.ll. NFC

-Don't use i64 for GEP indices.
-Remove tests that zero extended GEP indices from i32 to i64.
-Add i64 load/store tests taken from xtheadmemidx that get split into two i32 load/store.
-Don't extend load results to i64.

I'm working on improvements to SelectAddrRegRegScale.

Author: topperc