Fix: Compilation error when TM_MRG=0 or TM_AMVP=0 and redundant computation when --AltGPMSplitModeCode=0
(1) Fix compilation error when the macros TM_MRG and TM_AMVP are turned off. (2) Avoid encoder estimating reordered GPM split mode bit cost when GPM split mode reordering is off (that is --AltGPMSplitModeCode=0).