Rewrite BB args whose only use is in struct/tuple extract.

If we have BB args that are only used in a struct/tuple extract, and
that are generated in each predecessor with a struct/tuple instruction,
retype the BB arg and replace the argument with what would have been the
extracted value.

This provides more opportunties for jump threading to kick in.

Swift SVN r16509

lib/SIL/SILBasicBlock.cpp

@@ -72,6 +72,19 @@ void SILBasicBlock::eraseFromParent() {
   getParent()->getBlocks().erase(this);
 }
 
+/// Replace the ith BB argument with a new one with type Ty (and optional
+/// ValueDecl D).
+SILArgument *SILBasicBlock::replaceBBArg(unsigned i, SILType Ty, ValueDecl *D) {
+  SILModule &M = getParent()->getModule();
+  assert(BBArgList[i]->use_empty() && "Expected no uses of the old BB arg!");
+
+  auto *NewArg = new (M) SILArgument(Ty, D);
+  NewArg->setParent(this);
+  BBArgList[i] = NewArg;
+
+  return NewArg;
+}
+
 /// \brief Splits a basic block into two at the specified instruction.
 ///
 /// Note that all the instructions BEFORE the specified iterator