[gardening] Fix recently introduced "a" vs. "an" typos

Author: practicalswift

docs/SIL.rst

@@ -1217,7 +1217,7 @@ flow, such as a non-``Void`` function failing to ``return`` a value, or a
 The guaranteed dead code elimination pass can eliminate truly unreachable
 basic blocks, or ``unreachable`` instructions may be dominated by applications
 of functions returning uninhabited types. An ``unreachable`` instruction that
-survives guaranteed DCE and is not immediately preceded by an no-return
+survives guaranteed DCE and is not immediately preceded by a no-return
 application is a dataflow error.
 
 Runtime Failure

include/swift/SILOptimizer/Analysis/ARCAnalysis.h

@@ -396,7 +396,7 @@ namespace swift {
 /// EpilogueARCBlockState - Keep track of whether an epilogue ARC instruction
 /// has been found.
 struct EpilogueARCBlockState {
-  /// Keep track of whether a epilogue release has been found before and after
+  /// Keep track of whether an epilogue release has been found before and after
   /// this basic block.
   bool BBSetIn;
   /// The basic block local SILValue we are interested to find epilogue ARC in.