LetPropertyLowering: remove redundant phis after ssa-update

This is needed after running the SSAUpdater, because the updater can insert unnecessary phis in the middle of the original liverange of a value.

Fixes an ownership error.
rdar://153229472

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/FunctionPasses/LetPropertyLowering.swift

@@ -122,6 +122,8 @@ private func insertEndInitInstructions(
       use.set(to: ssaUpdater.getValue(atEndOf: use.instruction.parentBlock), context)
     }
   }
+  // This peephole optimization is required to avoid ownership errors.
+  replacePhisWithIncomingValues(phis: ssaUpdater.insertedPhis, context)
 }
 
 private func constructLetInitRegion(

test/SILOptimizer/let-property-lowering.sil

@@ -311,3 +311,34 @@ bb1(%3a : @reborrow $C):
   return %2 : $C
 }
 
+
+// CHECK-LABEL: sil [ossa] @test_no_phis :
+// CHECK:         %3 = end_init_let_ref %2
+// CHECK:         return %3
+// CHECK:       } // end sil function 'test_no_phis'
+sil [ossa] @test_no_phis : $@convention(thin) (Int, @owned C) -> @owned C {
+bb0(%0 : $Int, %1 : @owned $C):
+  %2 = mark_uninitialized [rootself] %1
+  %3 = begin_borrow %2
+  cond_br undef, bb1, bb2
+bb1:
+  br bb7
+bb2:
+  cond_br undef, bb3, bb8
+bb3:
+  cond_br undef, bb4, bb5
+bb4:
+  br bb7
+bb5:
+  br bb6
+bb6:
+  end_borrow %3
+  return %2
+bb7:
+  br bb6
+bb8:
+  end_borrow %3
+  destroy_value [dead_end] %2
+  unreachable
+}
+