Use getSCEV instead of getUnknown to create a SCEV for a

Use getSCEV instead of getUnknown to create a SCEV for a
Constant. This lets ConstantInts be handled as SCEVConstant instead
of SCEVUnknown, as getUnknown no longer has special-case code for
ConstantInt and friends. This usually doesn't affect the final
output, since the constants end up getting folded later, but it
does make intermediate expressions more obvious in many cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74459 91177308-0d34-0410-b5e6-96231b3b80d8