InstSimplify: Move a transform from InstCombine to InstSimplify

Several combines involving icmp (shl C2, %X) C1 can be simplified
without introducing any new instructions.  Move them to InstSimplify;
while we are at it, make them more powerful.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216642 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Clean up some tests to use FileCheck and combine two into a single
file.

Changing code that is covered by these tests is just too hard to debug
currently, and now it will be clear the nature of the changes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216643 91177308-0d34-0410-b5e6-96231b3b80d8