Fix mem2reg for load_borrows with reborrows

StackAllocationPromoter::pruneAllocStackUsage substitutes loads/stores of alloc_stack
with values. For some reason isLoadFromStack was bailing out for load_borrows with
reborrows leaving them to be fixed up by fixBranchesAndUses which uses live in value
from predecessors for substitution which is obviosly incorrect the block containing
the load_borrow has a store before it.

Fixes rdar://145834542

Author: meg-gupta

lib/SILOptimizer/Transforms/SILMem2Reg.cpp

@@ -377,11 +377,6 @@ static bool isLoadFromStack(SILInstruction *i, AllocStackInst *asi) {
   if (!isa<LoadInst>(i) && !isa<LoadBorrowInst>(i))
     return false;
 
-  if (auto *lbi = dyn_cast<LoadBorrowInst>(i)) {
-    if (BorrowedValue(lbi).hasReborrow())
-      return false;
-  }
-
   // Skip struct and tuple address projections.
   ValueBase *op = i->getOperand(0);
   while (op != asi) {

test/SILOptimizer/mem2reg_borrows.sil

@@ -408,3 +408,30 @@ bb6:
   %17 = tuple ()
   return %17 : $()
 }
+
+// CHECK-LABEL: sil [ossa] @test_load_borrow_with_reborrow :
+// CHECK-NOT: alloc_stack
+// CHECK-LABEL: } // end sil function 'test_load_borrow_with_reborrow'
+sil [ossa] @test_load_borrow_with_reborrow : $@convention(thin) () -> () {
+bb0:
+  %owned = apply undef() : $@convention(thin) () -> (@owned Klass)
+  %stack = alloc_stack [lexical] $Klass
+  store %owned to [init] %stack : $*Klass
+  br bb1
+
+bb1:
+  %ld = load [take] %stack : $*Klass
+  destroy_value %ld
+  %owned_other = apply undef() : $@convention(thin) () -> (@owned Klass)
+  store %owned_other to [init] %stack : $*Klass
+  %lb = load_borrow %stack
+  br bb2(%lb)
+
+bb2(%reborrow : @reborrow $Klass):
+  end_borrow %reborrow : $Klass
+  destroy_addr %stack : $*Klass
+  dealloc_stack %stack : $*Klass
+  %retval = tuple ()
+  return %retval : $()
+}
+