[NFC][InstCombine] Fine-tune 'check for [no] signed truncation' tests

We are using i8 for these tests, and shifting by 4,
which is exactly the half of i8.

But as it is seen from the proofs https://rise4fun.com/Alive/mgu
KeptBits = bitwidth(%x) - MaskedBits,
so with using shifts by 4, we are not really testing that
we actually properly handle the other cases with shifts not by half...

llvm-svn: 337208