### What changes were proposed in this pull request?

In `DivideYMInterval#doGenCode` and `DivideDTInterval#doGenCode`, rely on the operand variable names provided by `nullSafeCodeGen` rather than calling `genCode` on the operands twice.

### Why are the changes needed?

`DivideYMInterval#doGenCode` and `DivideDTInterval#doGenCode` call `genCode` on the operands twice (once directly, and once indirectly via `nullSafeCodeGen`). However, if you call `genCode` on an operand twice, you might not get back the same variable name for both calls (e.g., when the operand is not a `BoundReference` or if whole-stage codegen is turned off). When that happens, `nullSafeCodeGen` generates initialization code for one set of variables, but the divide expression generates usage code for another set of variables, resulting in compilation errors like this:
```
spark-sql> create or replace temp view v1 as
         > select * FROM VALUES
         > (interval '10' months, interval '10' day, 2)
         > as v1(period, duration, num);
Time taken: 2.81 seconds
spark-sql> cache table v1;
Time taken: 2.184 seconds
spark-sql> select period/(num + 3) from v1;
22/05/03 08:56:37 ERROR CodeGenerator: failed to compile: org.codehaus.commons.compiler.CompileException: File 'generated.java', Line 40, Column 44: Expression "project_value_2" is not an rvalue
...
22/05/03 08:56:37 WARN UnsafeProjection: Expr codegen error and falling back to interpreter mode
...
0-2
Time taken: 0.149 seconds, Fetched 1 row(s)
spark-sql> select duration/(num + 3) from v1;
22/05/03 08:57:29 ERROR CodeGenerator: failed to compile: org.codehaus.commons.compiler.CompileException: File 'generated.java', Line 40, Column 54: Expression "project_value_2" is not an rvalue
...
22/05/03 08:57:29 WARN UnsafeProjection: Expr codegen error and falling back to interpreter mode
...
2 00:00:00.000000000
Time taken: 0.089 seconds, Fetched 1 row(s)
```
The error is not fatal (unless you have `spark.sql.codegen.fallback` set to `false`), but it muddies the log and can slow the query (since the expression is interpreted).

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

New unit tests (unit tests run with `spark.sql.codegen.fallback` set to `false`, so the new tests fail without the fix).

Closes #36442 from bersprockets/interval_div_issue.

Authored-by: Bruce Robbins <bersprockets@gmail.com>
Signed-off-by: Gengliang Wang <gengliang@apache.org>
(cherry picked from commit ca87bead23ca32a05c6a404a91cea47178f63e70)
Signed-off-by: Gengliang Wang <gengliang@apache.org>