Pre-commit test case for trunc+lshr+load folds

This is a pre-commit of test cases relevant for D117406.

@srl_load_narrowing1 is showing a pattern that could be folded into
a more narrow load.

@srl_load_narrowing2 is showing a similar pattern that happens to
be optimized already, but that happens in two steps (first triggering
a combine based on SRL and later another combine based on TRUNCATE).

Differential Revision: https://reviews.llvm.org/D117588