The ARM backend did not expect LDRBi12 to hold a constant pool operand.
Allow for LLVM to deal with the instruction similar to how it deals with
LDRi12.
This fixes PR16215.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183238 91177308-0d34-0410-b5e6-96231b3b80d8