[MLIR][AArch64] Change some tests to ensure SVE vector length is the same throughout the function (#147506)
This change only applies to functions the can be reasonably expected to
use SVE registers.
Modifying vector length in the middle of a function might cause
incorrect stack deallocation if there are callee-saved SVE registers or
incorrect access to SVE stack slots.
Addresses (non-issue) https://github.com/llvm/llvm-project/issues/143670