[InstCombine] Fix out-of-bounds ashr(shl) optimization

This fixes a crash found by the oss fuzzer and reported by @fhahn.
The suggestion of @RKSimon seems to be the correct fix here. (See D91343).

The oss fuzz report can be found here: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=32759

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D99792