[InstCombine] improve demanded bits analysis of left-shifted operand
If we don't demand high bits, then we also don't care about those
high bits of a left-shift operand regardless of shift amount.
I noticed the sext/trunc pattern in a motivating example.
It seems like there should be a low-bits with right-shift sibling,
but I haven't looked at that yet.
Differential Revision: https://reviews.llvm.org/D101489