IRgen: Fix debug info regression in r106970; when we eliminate the return value

IRgen: Fix debug info regression in r106970; when we eliminate the return value
store make sure to move the debug metadata from the store (which is actual
'return' statement location) to the return instruction (which otherwise would
have the function end location as its debug info).
 - Tested by gdb test suite.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@107322 91177308-0d34-0410-b5e6-96231b3b80d8