[AArch64][GlobalISel] CallLowering: Don't generate new copies each time we need

to store to a stack location for outgoing args.

During call arg lowering we shouldn't be modifying SP so cache the SP copy
vreg for subsequent uses.

Gives a 0.2% geomean code size improvement on CTMark.

Differential Revision: https://reviews.llvm.org/D77838