[InstCombine] Support lshr(trunc(lshr(x,c1)), c2) ->...

[InstCombine] Support lshr(trunc(lshr(x,c1)), c2) -> trunc(lshr(lshr(x,c1),c2)) uniform vector tests

FoldShiftByConstant is hardcoded for scalar/uniform outer shift amounts atm so that needs to be fixed first to support non-uniform cases