[InstCombine] reassociateShiftAmtsOfTwoSameDirectionShifts(): fix miscompile (PR44802)

As input, we have the following pattern:
  Sh0 (Sh1 X, Q), K
We want to rewrite that as:
  Sh x, (Q+K)  iff (Q+K) u< bitwidth(x)
While we know that originally (Q+K) would not overflow
(because  2 * (N-1) u<= iN -1), we may have looked past extensions of
shift amounts. so it may now overflow in smaller bitwidth.

To ensure that does not happen, we need to ensure that the total maximal
shift amount is still representable in that smaller bitwidth.
If the overflow would happen, (Q+K) u< bitwidth(x) check would be bogus.

https://bugs.llvm.org/show_bug.cgi?id=44802
(cherry picked from commit 781d077a)