[AggressiveInstCombine] Fold strcmp for short string literals

Fold strcmp() against 1-char string literals.

This designates AggressiveInstCombine as the pass for libcalls
simplifications that may need to change the control flow graph.

Fixes https://github.com/llvm/llvm-project/issues/58003.

Differential Revision: https://reviews.llvm.org/D154725