Missed Optimization: hoist GEP out of select by selecting the offset

```
define ptr @src(i8 %arg1, ptr %arg0) {
%v0 = icmp eq i8 %arg1, 116
%v1 = select i1 %v0, ptr %arg0, ptr null
%v2 = getelementptr inbounds nuw i8, ptr %v1, i16 48
%v3 = getelementptr inbounds nuw i8, ptr %arg0, i16 40
%v4 = select i1 %v0, ptr %v2, ptr %v3
ret ptr %v4
}

define ptr @tgt(i8 %arg1, ptr %arg0) {
%v0 = icmp eq i8 %arg1, 116
%v1 = select i1 %v0, i16 48, i16 40
%v2 = getelementptr inbounds i8, ptr %arg0, i16 %v1
ret ptr %v2
}
```
alive2:https://alive2.llvm.org/ce/z/YNjzLG
godbolt:https://godbolt.org/z/Tovh8vzvv
Pattern found in: https://github.com/dtcxzyw/llvm-opt-benchmark/blob/main/bench/llvm/optimized/CheckExprLifetime.ll

Start with the LLVM IR example in the issue and compare the source and target functions using the linked Alive2 proof and Godbolt reproduction. Read the referenced llvm-opt-benchmark pattern in bench/llvm/optimized/CheckExprLifetime.ll, then trace the relevant LLVM optimization behavior. Done means the missed optimization is recognized and the resulting code matches the target transformation.

Written by the indexing model from the issue text.