AArch64: Simplify logic in deciding whether bfi is valid

Previous code had a confusing comment which was mostly an implementation
detail. This condition corresponds to "lsb up to register width" and "width not
ridiculous".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174877 91177308-0d34-0410-b5e6-96231b3b80d8