[GlobalISel] Fix sext_inreg(load) combine to not move the originating load.

The builder was using the extend user as the insertion point, which meant that
we were incorrectly "moving" the load from its original position, and therefore
could violate memory operation ordering.

lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -730,7 +730,7 @@ bool CombinerHelper::applySextInRegOfLoad(
   // %ld = G_SEXTLOAD %ptr (load 1)
 
   auto &MMO = **LoadDef->memoperands_begin();
-  Builder.setInstrAndDebugLoc(MI);
+  Builder.setInstrAndDebugLoc(*LoadDef);
   auto &MF = Builder.getMF();
   auto PtrInfo = MMO.getPointerInfo();
   auto *NewMMO = MF.getMachineMemOperand(&MMO, PtrInfo, ScalarSizeBits / 8);

test/CodeGen/AArch64/GlobalISel/prelegalizercombiner-sextload-from-sextinreg.mir

@@ -24,6 +24,34 @@ body:             |
     $w0 = COPY %3(s32)
     RET_ReallyLR implicit $w0
 
+...
+---
+name:            sextload_from_inreg_across_store
+alignment:       4
+tracksRegLiveness: true
+liveins:
+  - { reg: '$x0' }
+body:             |
+  bb.1:
+    liveins: $x0
+    ; Check that the extend gets folded into the load, not the other way around, which
+    ; could cause mem dependence violations.
+    ; CHECK-LABEL: name: sextload_from_inreg_across_store
+    ; CHECK: liveins: $x0
+    ; CHECK: [[COPY:%[0-9]+]]:_(p0) = COPY $x0
+    ; CHECK: [[SEXTLOAD:%[0-9]+]]:_(s16) = G_SEXTLOAD [[COPY]](p0) :: (load 1, align 2)
+    ; CHECK: G_STORE [[COPY]](p0), [[COPY]](p0) :: (store 8)
+    ; CHECK: [[ANYEXT:%[0-9]+]]:_(s32) = G_ANYEXT [[SEXTLOAD]](s16)
+    ; CHECK: $w0 = COPY [[ANYEXT]](s32)
+    ; CHECK: RET_ReallyLR implicit $w0
+    %0:_(p0) = COPY $x0
+    %1:_(s16) = G_LOAD %0(p0) :: (load 2)
+    G_STORE %0(p0), %0(p0) :: (store 8)
+    %2:_(s16) = G_SEXT_INREG %1, 8
+    %3:_(s32) = G_ANYEXT %2(s16)
+    $w0 = COPY %3(s32)
+    RET_ReallyLR implicit $w0
+
 ...
 ---
 name:            non_pow_2_inreg