[NFC][InstCombine] Some tests for 'shift amount reassoc in bit test - trunc-of-lshr' (PR42399)

Finally, the fold i was looking forward to :)

The legality check is muddy, i doubt  i've groked the full generalization,
but it handles all the cases i care about, and can come up with:
https://rise4fun.com/Alive/26j

https://bugs.llvm.org/show_bug.cgi?id=42399

llvm-svn: 369197