ARM: remove unnecessary state-tracking during frame lowering.
ResolveFrameIndex had what appeared to be a very nasty hack for when the frame-index referred to a callee-saved register. In this case it "adjusted" the offset so that the address was correct if (and only if) the MachineInstr immediately followed the respective push. This "worked" for all forms of GPR & DPR but was only ever used to set the frame pointer itself, and once this was put in a more sensible location the entire state-tracking machinery it relied on became redundant. So I stripped it. The only wrinkle is that "add r7, sp, #0" might theoretically be slower (need an actual ALU slot) compared to "mov r7, sp" so I added a micro-optimisation that also makes emitARMRegUpdate and emitT2RegUpdate also work when NumBytes == 0. No test changes since there shouldn't be any functionality change. llvm-svn: 194025
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