cmd/compile: reject some rare looping CFGs in shortcircuit
One CFGs that shortcircuit looks for is:
p q
\ /
b
/ \
t u
The test case creates a CFG like that in which p == t.
That caused the compiler to generate a (short-lived) invalid phi value.
Fix this with a relatively big hammer: Disallow single-length loops entirely.
This is probably overkill, but it such loops are very rare.
This doesn't change the generated code for anything in std.
It generates worse code for the test case:
It no longer compiles the entire function away.