[NFC][InstCombine] Redundant masking before left-shift: tests with assume

If the legality check is `(shiftNbits-maskNbits) s>= 0`,
then we can simplify it to `shiftNbits u>= maskNbits`,
which is easier to check for.

However, currently switching the `dropRedundantMaskingOfLeftShiftInput()`
to `SimplifyICmpInst()` does not catch these cases and regresses
currently-handled cases, so i'll leave it as is for now.

https://rise4fun.com/Alive/25P

llvm-svn: 366564