Fix compound assignment

[tgrant-nv]

Description

This change removes the de-sugaring of a += b; into a = a + b; that was happening very early during the AST construction, and instead makes the generation of OSO IR make sure the a side is evaluated only once.

This is because the old behavior wouldn't handle correctly uses like a[i++] += b[j++]; (which is the array form of a += b; in a way). Because the old behavior would evaluate the a side twice, it would increment i twice, summing the elements of b to the even elements of a, and possibly accessing outside of the bounds of a.

Tests

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format v17 before submitting, I definitely will look at
  the CI test that runs clang-format and fix anything that it highlights as
  being nonconforming.

[tgrant-nv]

Luca and I will look into these CI failures and update the PR ASAP. This is not a failure we saw in our testing, but I also did not enable batching.