Clarify and make explicit some of the requirements for transforming

Clarify and make explicit some of the requirements for transforming
mask+shift pairs at the beginning of the ISD::AND case block, and then
hoist the final pattern into a helper function, simplifying and
reflowing it appropriately. This should have no observable behavior
change, but several simplifications fell out of this such as directly
computing the new mask constant, etc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147939 91177308-0d34-0410-b5e6-96231b3b80d8