[GISel] Add debug counter to force sdag fallback (#78257)

Add a debug counter that allows forcing an sdag fallback after a certain
number of functions.

The intended use-case is to bisect which function gets miscompiled by
global isel using `-debug-counter=globalisel-count=N` (in cases where
sdag doesn't also miscompile it, of course).

The "falling back" debug line is printed unconditionally, because using
`-debug-only` is usually too spammy for the intended purpose.