[AArch64] Remove dead code for inserting SEH opcodes for realignment. NFC.

[mstorsjo]

Summary

If the stack is realigned, we've emitted a frame pointer and
already terminated the SEH prologue, making this dead code since
a07787c9a50c046e45921dd665f5a53a752bbc31.

The immediate to this SEH opcode was entirely bogus - we don't
know how many bytes the AND operation adjusts the SP, and by
doing "NumBytes & andMaskEncoded" (where andMaskEncoded was the
immediate bitpattern for the AND instruction), the immediate to the
opcode was total gibberish.

This hasn't had any practical effect, since the original stack
pointer always was restored from the frame pointer afterwards anyway.

[efriedma]

LGTM

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

if (NeedsRealignment) {
       uint32_t andMaskEncoded = (1 << 12)                         // = N
                                 | ((64 - NrBitsToZero) << 6)      // immr
                                 | ((64 - NrBitsToZero - 1) << 0); // imms
 
       BuildMI(MBB, MBBI, DL, TII->get(AArch64::ANDXri), AArch64::SP)
           .addReg(scratchSPReg, RegState::Kill)
           .addImm(andMaskEncoded);
       AFI->setStackRealigned(true);
-      if (NeedsWinCFI) {
-        HasWinCFI = true;
-        BuildMI(MBB, MBBI, DL, TII->get(AArch64::SEH_StackAlloc))
-            .addImm(NumBytes & andMaskEncoded)
-            .setMIFlag(MachineInstr::FrameSetup);
-      }
+
+      // No need for SEH instructions here; if we're realigning the stack,
+      // we've set a frame pointer and already finished the SEH prologue.
+      assert(!NeedsWinCFI);
     }
   }
 
   // If we need a base pointer, set it up here. It's whatever the value of the
   // stack pointer is at this point. Any variable size objects will be allocated
   // after this, so we can still use the base pointer to reference locals.
   //
   // FIXME: Clarify FrameSetup flags here.