[ConstantRange] Optimize APInt creation in getSignedMax/getSignedMin.

We were creating an APInt at the top of these methods that isn't always returned. For ranges wider than 64-bits this results in an allocation and deallocation when its not used.

In getSignedMax we were creating Upper-1 to use in a compare and then creating it again for a return value. The compiler is unable to determine that these can be shared. So help it out and create the Upper-1 in a temporary that can be reused.

This provides a little compile time improvement.

llvm-svn: 300621