Improved fix for PR17827 (instcombine of shift/and/compare).

This change fixes the case of arithmetic shift right - do not attempt to fold that case.
This change also relaxes the conditions when attempting to fold the logical shift right and shift left cases.

No additional IR-level test cases included at this time. See http://llvm.org/bugs/show_bug.cgi?id=17827 for proofs that these are correct transformations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197705 91177308-0d34-0410-b5e6-96231b3b80d8