Implement getSubRegIndex as a linear search on the SubRegTable instead of using a...

Implement getSubRegIndex as a linear search on the SubRegTable instead of using a big switch.

- The search bounds are constant, in the worst case (ARM target) it will scan over 30 uint16_ts.
- This method isn't very hot, I had problems finding a testcase where it's called more than a dozen of times (no perf impact).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151773 91177308-0d34-0410-b5e6-96231b3b80d8