[CanonicalizeInst] Remove redundant lexical bbis.

Previously, CanonicalizeInstruction::eliminateSimpleBorrows bailed out
when encountering any any [lexical] begin_borrow.  While generally such
borrow scopes must be preserved, they may be removed if they are
redundant (when the borrowed value is guaranteed by another means: an
outer lexical borrow scope or a guaranteed function argument).  Here,
removing such redundant lexical borrow scopes is enabled.

lib/SILOptimizer/Utils/CanonicalizeInstruction.cpp

@@ -445,9 +445,13 @@ static SILBasicBlock::iterator
 eliminateSimpleBorrows(BeginBorrowInst *bbi, CanonicalizeInstruction &pass) {
   auto next = std::next(bbi->getIterator());
 
-  // Never eliminate lexical borrow scopes.  They must be kept to ensure that
-  // value lifetimes aren't observably shortened.
-  if (bbi->isLexical())
+  // Lexical borrow scopes can only be eliminated under certain circumstances:
+  // (1) They can never be eliminated if the module is in the raw stage, because
+  //     they may be needed for diagnostic.
+  // (2) They can never be eliminated if there is no enclosing lexical scope
+  //     which guarantees the lifetime of the value.
+  if (bbi->isLexical() && (bbi->getModule().getStage() == SILStage::Raw ||
+                           !isNestedLexicalBeginBorrow(bbi)))
     return next;
 
   // We know that our borrow is completely within the lifetime of its base value