ARMInstrInfo: Improve isSwiftFastImmShift

An instruction with less than 3 inputs is trivially a fast immediate shift.

Reapply of 183256, should not have caused the tablegen segfault on linux either.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183314 91177308-0d34-0410-b5e6-96231b3b80d8