[InstCombine] dropRedundantMaskingOfLeftShiftInput(): check that adding shift...

[InstCombine] dropRedundantMaskingOfLeftShiftInput(): check that adding shift amounts doesn't overflow (PR49778)

This is identical to 781d077a,
but for the other function.

For certain shift amount bit widths, we must first ensure that adding
shift amounts is safe, that the sum won't have an unsigned overflow.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49778