ART: Clear use count for unused VRs.

The use count of temp VR should be cleared when we replace
"CMP_XXX vA, vB, vC" and "IF_XXX vA" with "kMirOpFusedCmpXXX vB, vC".
Otherwise, the backend may allocate a physical register for the unused vA.

Change-Id: I43ad37d0e7161ec3de154de8888caa94603f7715
1 file changed