[X86][VARARG] Assign MMO earlier to avoid prolog insert point been sunk across...

[X86][VARARG] Assign MMO earlier to avoid prolog insert point been sunk across VASTART_SAVE_XMM_REGS

The changes in D80163 defered the assignment of MachineMemOperand (MMO)
until the X86ExpandPseudo pass. This will result in crash due to prolog
insert point been sunk across the pseudo instruction VASTART_SAVE_XMM_REGS.

Moving the assignment to the creation of the node can avoid the problem.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D112859