Fix pr6111: Avoid using the LR register for the target address of an indirect

Fix pr6111: Avoid using the LR register for the target address of an indirect
branch in ARM v4 code, since it gets clobbered by the return address before
it is used.  Instead of adding a new register class containing all the GPRs
except LR, just use the existing tGPR class.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@96360 91177308-0d34-0410-b5e6-96231b3b80d8