[X86 disasm tblegen backend] Clean up numPhysicalOperands asserts

No functionality change intended.

This implements Elena's idea to put the new additionalOperand outside the
switch to cover all cases
(http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140929/237763.html).

Note only nontrivial change is in MRMSrcMemFrm.  This requires an inclusive
interval of [2, 4] because we have prefix-dependent *optional* immediate
operand.