[InstCombine] reduce multi-use casts+masks

As noted in the code comment, we could generalize this:
https://alive2.llvm.org/ce/z/N5m-eZ

It saves an instruction even without a constant operand,
but the 'and' is wider. We can do that as another step
if it doesn't harm anything.

I noticed that this missing pattern with a constant operand
inhibited other transforms in a recent bug report, so this
is enough to solve that case.