Fix wrong encoding of MRSBanked.

Patch by Matthew Wahab.

Change-Id: Ia2a001ca2760028ea360fe77b56f203a219eefbc

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222920 91177308-0d34-0410-b5e6-96231b3b80d8