This libgo patch adds a missing type conversion.  The gofrontend
incorrectly accepted code that was missing a type conversion.  The
test case for this is bug518.go in https://go.dev/cl/536537.  Future
CLs in this series will detect the type error.  Bootstrapped and ran
Go testsuite on x86_64-pc-linux-gnu.  Committed to mainline.

Ian