[llvm] [RISCV] Narrow indices of fixed vector gather/scatter nodes (PR #66405)

[Philip Reames via llvm-commits]

preames wrote:

Since github makes this really hard to follow, let me spell out that the three changes approved by @topperc above were landed as the following:

```
commit 37aa07ad312f58bd2ca59ea712a72544f753f5d3
Author: Philip Reames <preames at rivosinc.com>
Date:   Wed Sep 13 14:56:15 2023 -0700

    [RISCV] Move narrowIndex to be a DAG combine over target

commit 2ff9175af73eb4df7bd2fda35591a95e0c6ba236
Author: Philip Reames <preames at rivosinc.com>
Date:   Thu Sep 14 09:58:52 2023 -0700

    [RISCV] Normalize gather/scatter addressing to UNSIGNED_SCALAR

commit 09a5aac514fa85cb202bfb2faa22ed6b7a29d94a
Author: Philip Reames <preames at rivosinc.com>
Date:   Wed Sep 13 14:39:08 2023 -0700

    [TLI] Add extend as explicit parameter to shouldRemoveExtendFromGSIndex [nfc]

```
I have not yet addressed the changes requested for the final change, and will do so shortly.  

https://github.com/llvm/llvm-project/pull/66405