[LowerSwitch] Use ConstantRange::fromKnownBits(); NFC

Using an unsigned range to stay NFC, but a signed range would really
be more useful here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356831 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed