[AMDGPU] Fix isReallyTriviallyReMaterializable for V_MOV_*

D57708 changed SIInstrInfo::isReallyTriviallyReMaterializable to reject
V_MOVs with extra implicit operands, but it accidentally rejected all
V_MOVs because of their implicit use of exec. Fix it but avoid adding a
moderately expensive call to MI.getDesc().getNumImplicitUses().

In real graphics shaders this changes quite a few vgpr copies into move-
immediates, which is good for avoiding stalls on GFX10.

Differential Revision: https://reviews.llvm.org/D98347