[CodeGen] Fix lowering for returning the result of an extractvalue

When the number of return values exceeds the number of registers available,
SelectionDAGBuilder::visitRet transforms a function's return to use a
pointer to a buffer to hold return values. When the returned value is an
operator such as extractvalue, the value may have a non-zero result number.
Add that number to the indexing when obtaining the values to store.

This fixes https://bugs.llvm.org/show_bug.cgi?id=43132.

Differential Revision: https://reviews.llvm.org/D66978

llvm-svn: 370430