[InstCombine] move folds for shift-shift pairs; NFCI

Although this is 'no-functional-change-intended', I'm adding tests
for shl-shl and lshr-lshr pairs because there is no existing test 
coverage for those folds.

It seems like we should be able to remove some code from foldShiftedShift()
at this point because we're handling those patterns on the general path.

llvm-svn: 293814