This patch to the Go frontend loads LHS subexpressions of op=
assignment only once.  This avoids inconsistencies if the variables
are changed by evaluating the RHS.  This fixes
https://go.dev/issue/52811.  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline.  There is a test case in
the main Go repo that will be copied over at some point.

Ian