[InstCombine] improve matching for sext-lshr-trunc patterns

This is intended to preserve the logic of the existing transform,
but remove unnecessary restrictions on uses and types.

https://rise4fun.com/Alive/pYfR

  Pre: C1 <= width(C1) - 8
  %B = sext i8 %A
  %C = lshr %B, C1
  %r = trunc %C to i8
   =>
  %r = ashr i8 %A, trunc(umin(C1, 7))