interp: bail out of loops that iterate too many times (#5395)

* interp: bail out of loops that iterate too many times

The existing loop guard (errLoopUnrolled) only fires when a loop body
emits runtime instructions. Loops that are fully evaluable at compile
time, such as inserting thousands of entries into a map, were not
caught and could hang the compiler.

Add a per-basic-block iteration counter that triggers a recoverable
error (errLoopTooLong) when any block is entered more than 1000 times
in a single function call. This defers the init function to runtime,
which is the same behavior as other interp bailouts.

Profiling showed that 83% of CPU time was spent in GC, caused by
allocation pressure from the interp memory cloning on each map
mutation. The iteration limit avoids this entirely by bailing out
before the quadratic cost becomes significant.

Performance on the reproducer from #2090 (map init with strconv.Itoa):

    entries  before       after
    5,000    7.4s         2.1s
    10,000   17.5s        2.1s
    20,000   48.0s        2.8s
    65,536   >180s (OOM)  3.2s

* Add -interp-loop-limit

interp/errors.go

@@ -19,6 +19,7 @@ var (
 	errUnsupportedRuntimeInst = errors.New("interp: unsupported instruction (to be emitted at runtime)")
 	errMapAlreadyCreated      = errors.New("interp: map already created")
 	errLoopUnrolled           = errors.New("interp: loop unrolled")
+	errLoopTooLong            = errors.New("interp: loop ran too many iterations")
 )
 
 // This is one of the errors that can be returned from toLLVMValue when the
@@ -29,7 +30,7 @@ var errInvalidPtrToIntSize = errors.New("interp: ptrtoint integer size does not
 func isRecoverableError(err error) bool {
 	return err == errIntegerAsPointer || err == errUnsupportedInst ||
 		err == errUnsupportedRuntimeInst || err == errMapAlreadyCreated ||
-		err == errLoopUnrolled || err == errInvalidPtrToIntSize
+		err == errLoopUnrolled || err == errLoopTooLong || err == errInvalidPtrToIntSize
 }
 
 // ErrorLine is one line in a traceback. The position may be missing.