[InstCombine] Right-shift shift amount reassociation with truncation (PR43564, PR42391)

Initially (D65380) i believed that if we have rightshift-trunc-rightshift,
we can't do any folding. But as it usually happens, i was wrong.

https://rise4fun.com/Alive/GEw
https://rise4fun.com/Alive/gN2O

In https://bugs.llvm.org/show_bug.cgi?id=43564 we happen to have
this very sequence, of two right shifts separated by trunc.
And "just" so that happens, we apparently can fold the pattern
if the total shift amount is either 0, or it's equal to the bitwidth
of the innermost widest shift - i.e. if we are left with only the
original sign bit. Which is exactly what is wanted there.

llvm-svn: 373801