[AArch64][GlobalISel] Protect against folding loads across basic blocks.

isObviouslySafeToFold can look between a load and an instruction it can be
folded into, to check that no other memory operations prevents the fold. It
doesn't handle multiple basic blocks which we needs to guard against.

Author: davemgreen

llvm/lib/CodeGen/GlobalISel/GIMatchTableExecutor.cpp

@@ -98,5 +98,5 @@ bool GIMatchTableExecutor::isObviouslySafeToFold(MachineInstr &MI,
     return true;
   }
 
-  return true;
+  return !MI.mayLoad();
 }

llvm/test/CodeGen/AArch64/neon-vector-splat.ll

@@ -126,24 +126,15 @@ define <2 x ptr> @shuffle8(ptr %P) {
 }
 
 define <4 x i32> @multiblock_aliasing(ptr %P, i1 %c) {
-; CHECK-SD-LABEL: multiblock_aliasing:
-; CHECK-SD:       // %bb.0: // %entry
-; CHECK-SD-NEXT:    ldr q0, [x0]
-; CHECK-SD-NEXT:    tbz w1, #0, .LBB9_2
-; CHECK-SD-NEXT:  // %bb.1: // %then
-; CHECK-SD-NEXT:    stp xzr, xzr, [x0]
-; CHECK-SD-NEXT:  .LBB9_2: // %else
-; CHECK-SD-NEXT:    dup v0.4s, v0.s[0]
-; CHECK-SD-NEXT:    ret
-;
-; CHECK-GI-LABEL: multiblock_aliasing:
-; CHECK-GI:       // %bb.0: // %entry
-; CHECK-GI-NEXT:    tbz w1, #0, .LBB9_2
-; CHECK-GI-NEXT:  // %bb.1: // %then
-; CHECK-GI-NEXT:    stp xzr, xzr, [x0]
-; CHECK-GI-NEXT:  .LBB9_2: // %else
-; CHECK-GI-NEXT:    ld1r { v0.4s }, [x0]
-; CHECK-GI-NEXT:    ret
+; CHECK-LABEL: multiblock_aliasing:
+; CHECK:       // %bb.0: // %entry
+; CHECK-NEXT:    ldr q0, [x0]
+; CHECK-NEXT:    tbz w1, #0, .LBB9_2
+; CHECK-NEXT:  // %bb.1: // %then
+; CHECK-NEXT:    stp xzr, xzr, [x0]
+; CHECK-NEXT:  .LBB9_2: // %else
+; CHECK-NEXT:    dup v0.4s, v0.s[0]
+; CHECK-NEXT:    ret
 entry:
   %lv2ptr = load <4 x i32>, ptr %P
   br i1 %c, label %then, label %else