[Thumb] Don't try and emit LDRH/LDRB from the constant pool

This is not a valid encoding - these instructions cannot do PC-relative addressing.

The underlying problem here is of whitelist in ARMISelDAGToDAG that unwraps ARMISD::Wrappers during addressing-mode selection. This didn't realise TargetConstantPool was actually possible, so didn't handle it.

llvm-svn: 283323