[InstCombine] Combine const GEP chains

This patch reverts rGae739aefd7473517d3f08b5c8d08a66c7f469198 to address performance regressions reported by our [CI](https://github.com/dtcxzyw/llvm-ci/issues/137) after rG2ec1d0f427c7822540352c0c14d057e7bfe4f77b.

For example:
```
define ptr @const_gep_chain(ptr %p, i64 %a) {
    %p1 = getelementptr inbounds i8, ptr %p, i64 %a
    %p2 = getelementptr inbounds i8, ptr %p1, i64 1
    %p3 = getelementptr inbounds i8, ptr %p2, i64 2
    %p4 = getelementptr inbounds i8, ptr %p3, i64 3
    ret ptr %p4
}
```
The last three GEPs will not be folded since rG2ec1d0f427c7822540352c0c14d057e7bfe4f77b.

I think it is appropriate to remove this code because there is no compile-time regression reported in our benchmarks.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D149240