[AMDGPU] GCNDPPCombine: don't shrink V_ADD_CO_U32 if carry out is used

Don't shrink VOP3 instructions if there are any uses of a carry-out
operand, because the shrunken form of the instruction would write the
carry-out to vcc instead of to a virtual register.

Differential Revision: https://reviews.llvm.org/D100760