[VectorCombine] Fix load extract scalarization tests with assumes.

The input IR for @load_extract_idx_var_i64_known_valid_by_assume
and @load_extract_idx_var_i64_not_known_valid_by_assume_after_load
has been swapped.

This patch fixes the test so that @load_extract_idx_var_i64_known_valid_by_assume
has the assume before the load and the other test has it after.