[AMDGPU] Set SchedRW = Write64Bit on V_MOV_B64 (llvm#70135)

This is mostly NFCI as V_MOV_B64_PSEUDO is selected instead and expanded
past RA.

Author: rampitec

llvm/lib/Target/AMDGPU/VOP1Instructions.td

@@ -221,7 +221,7 @@ def VOPProfile_MOV : VOPProfile <[i32, i32, untyped, untyped]> {
 let isReMaterializable = 1, isAsCheapAsAMove = 1 in {
 defm V_MOV_B32 : VOP1Inst <"v_mov_b32", VOPProfile_MOV, null_frag, 0x8>;
 
-let SubtargetPredicate = isGFX940Plus in
+let SubtargetPredicate = isGFX940Plus, SchedRW = [Write64Bit] in
 defm V_MOV_B64 : VOP1Inst <"v_mov_b64", VOP_I64_I64>;
 } // End isMoveImm = 1