Lower X86::MORESTACK_RET and X86::MORESTACK_RET_RESTORE_R10 in
X86AsmPrinter::EmitInstruction, rather than X86MCInstLower::Lower.

The aim is to improve the reusability of the X86MCInstLower class by making it
more function-like. The X86::MORESTACK_RET_RESTORE_R10 pseudo broke the
function model by emitting an extra instruction to the MCStreamer attached to
the AsmPrinter.

The patch should have no impact on generated code.