[GlobalISel] Check the correct register in sextload OneUse check. (#114763)

This fixes a bug that started triggering after #111730, where we could
remove a load with multiple uses. It looks like the match should be
checking the other register in a one-use check.

  %SrcReg = load..
  %DstReg = sign_extend_inreg %SrcReg

Author: davemgreen

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

@@ -1049,7 +1049,7 @@ bool CombinerHelper::matchSextInRegOfLoad(
 
   Register SrcReg = MI.getOperand(1).getReg();
   auto *LoadDef = getOpcodeDef<GLoad>(SrcReg, MRI);
-  if (!LoadDef || !MRI.hasOneNonDBGUse(DstReg))
+  if (!LoadDef || !MRI.hasOneNonDBGUse(SrcReg))
     return false;
 
   uint64_t MemBits = LoadDef->getMemSizeInBits().getValue();

llvm/test/CodeGen/AArch64/load.ll

@@ -465,3 +465,17 @@ define <2 x fp128> @load_v2f128(ptr %p) {
   %a = load <2 x fp128>, ptr %p
   ret <2 x fp128> %a
 }
+
+define i32 @load_i8_s16_extrasuse(ptr %ptr, ptr %ptr2) {
+; CHECK-LABEL: load_i8_s16_extrasuse:
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    ldr w8, [x0]
+; CHECK-NEXT:    sxtb w0, w8
+; CHECK-NEXT:    str w8, [x1]
+; CHECK-NEXT:    ret
+  %a = load i32, ptr %ptr
+  %s = shl i32 %a, 24
+  %b = ashr i32 %s, 24
+  store i32 %a, ptr %ptr2
+  ret i32 %b
+}