ARM: restrict register class for WIN__DBZCHK

WIN__DBZCHK will insert a CBZ instruction into the stream.  This instruction
reserves 3 bits for the condition register (rn).  As such, we must ensure that
we restrict the register to a low register.  Use the tGPR class instead of GPR
to ensure that this is properly constrained.  In debug builds, we would attempt
to use lr as a condition register which would silently get truncated with no
hint that the register selection was incorrect.

llvm-svn: 267080