Fix icmp lowering

Fix icmp lowering

Summary:
During icmp lowering it can happen that a constant value can be larger than expected (see the code around the change).
APInt::getMinSignedBits() must be checked again as the shift before can change the constant sign to positive.
I'm not sure it is the best fix possible though.

Test Plan: Regression test included.

Reviewers: resistor, chandlerc, spatel, hfinkel

Reviewed By: hfinkel

Subscribers: hfinkel, llvm-commits

Differential Revision: http://reviews.llvm.org/D9147

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237812 91177308-0d34-0410-b5e6-96231b3b80d8