[llvm] [InstCombine] Teach foldSelectOpOp about samesign (PR #122723)

[via llvm-commits]

asmok-g wrote:

A bit of an incomplete hint; I think the most interesting diff in IR is:
there are two missing `add`s before icmp
```
-  %195 = ashr exact i32 %194, 13
-  %196 = add nsw i32 %195, -131072
-  %197 = icmp ult i32 %196, -262144
...
+  %194 = ashr exact i32 %193, 13
+  %195 = icmp ugt i32 %194, 131071
``` 
and 
```
-  %111 = add nsw i32 %110, -131072
-  %112 = icmp ult i32 %111, -262144
-  %113 = select i1 %112, i32 0, i32 %110
+  %111 = icmp ugt i32 %110, 131071
+  %112 = select i1 %111, i32 0, i32 %110
```

https://github.com/llvm/llvm-project/pull/122723