[X86][AsmParser] Refactor code and optimize more instructions from VEX3 to VEX2

1. Share code `optimizeInstFromVEX3ToVEX2` with MCInstLower
2. Move the code of optimization for shift/rotate to a separate file
3. Since the function is shared, a side effect is that more encoding
   optimizations are done on the Asmparser side. Considering we already
   use reverse-encoding for optimization in AsmParser before this patch,
   I believe the change is positive and expected.

This is a reland of D150068 with the fix D150440.