gc: fix label recursion bugs

gc: fix label recursion bugs

Was keeping a pointer to the labeled statement in n->right,
which meant that generic traversals of the tree visited it twice.
That combined with aggressive flattening of the block
structure when possible during parsing meant that
the kinds of label: code label: code label: code sequences
generated by yacc were giving the recursion 2ⁿ paths
through the program.

Fixes #2212.

R=lvd
CC=golang-dev
https://golang.org/cl/4960050