[X86] Add MI-layer routine for getting the index of the first address operand, NFC (#78019)

Add the MI-layer routine X86::getFirstAddrOperandIdx(), which returns
the index of the first address operand of a MachineInstr (or -1 if there
is none).

X86II::getMemoryOperandNo(), the existing MC-layer routine used to
obtain the index of the first address operand in a 5-operand X86 memory
reference, is incomplete: it does not handle pseudo-instructions like
TCRETURNmi, resulting in security holes in the mitigation passes that
use it (e.g., x86-slh and x86-lvi-load).

X86::getFirstAddrOperandIdx() handles both pseudo and real instructions
and is thus more suitable for most use cases than
X86II::getMemoryOperandNo(), especially in mitigation passes like
x86-slh and x86-lvi-load. For this reason, this patch replaces all uses
of X86II::getMemoryOperandNo() with X86::getFirstAddrOperandIdx() in the
aforementioned mitigation passes.