[MachineLICM] Don't allow hoisting invariant loads across mem barrier. (#116987)

The improvements in 63917e1 / #70796 do not check for memory
barriers/unmodelled sideeffects, which means we may incorrectly hoist
loads across memory barriers.

Fix this by checking any machine instruction in the loop is a load-fold
barrier.

PR: #116987

Author: fhahn

llvm/lib/CodeGen/MachineLICM.cpp

@@ -1501,7 +1501,7 @@ void MachineLICMImpl::InitializeLoadsHoistableLoops() {
       if (!AllowedToHoistLoads[Loop])
         continue;
       for (auto &MI : *MBB) {
-        if (!MI.mayStore() && !MI.isCall() &&
+        if (!MI.isLoadFoldBarrier() && !MI.mayStore() && !MI.isCall() &&
             !(MI.mayLoad() && MI.hasOrderedMemoryRef()))
           continue;
         for (MachineLoop *L = Loop; L != nullptr; L = L->getParentLoop())

llvm/test/CodeGen/AArch64/machine-licm-hoist-load.ll

@@ -499,16 +499,16 @@ for.exit:                                 ; preds = %for.body
 
 @a = external local_unnamed_addr global i32, align 4
 
-; FIXME: Load hoisted out of the loop across memory barriers.
+; Make sure the load is not hoisted out of the loop across memory barriers.
 define i32 @load_between_memory_barriers() {
 ; CHECK-LABEL: load_between_memory_barriers:
 ; CHECK:       // %bb.0:
 ; CHECK-NEXT:    adrp x8, :got:a
 ; CHECK-NEXT:    ldr x8, [x8, :got_lo12:a]
-; CHECK-NEXT:    ldr w0, [x8]
 ; CHECK-NEXT:  .LBB8_1: // %loop
 ; CHECK-NEXT:    // =>This Inner Loop Header: Depth=1
 ; CHECK-NEXT:    //MEMBARRIER
+; CHECK-NEXT:    ldr w0, [x8]
 ; CHECK-NEXT:    //MEMBARRIER
 ; CHECK-NEXT:    cbz w0, .LBB8_1
 ; CHECK-NEXT:  // %bb.2: // %exit

llvm/test/CodeGen/Mips/lcb5.ll

@@ -186,7 +186,7 @@ if.end:                                           ; preds = %if.then, %entry
 }
 
 ; ci:	.ent	z3
-; ci:	bteqz	$BB6_3
+; ci:	bteqz	$BB6_2
 ; ci:	.end	z3
 
 ; Function Attrs: nounwind optsize
@@ -210,7 +210,7 @@ if.end:                                           ; preds = %if.then, %entry
 
 ; ci:	.ent	z4
 ; ci:	btnez	$BB7_1  # 16 bit inst
-; ci:	jal	$BB7_3	# branch
+; ci:	jal	$BB7_2	# branch
 ; ci:	nop
 ; ci: $BB7_1:
 ; ci:	.p2align	2