runtime: sample large heap allocations correctly

Remove an unnecessary check on the heap sampling code that forced sampling
of all heap allocations larger than the sampling rate. This need to follow
a poisson process so that they can be correctly unsampled. Maintain a check
for MemProfileRate==1 to provide a mechanism for full sampling, as
documented in https://golang.org/pkg/runtime/#pkg-variables.

Additional testing for this change is on cl/129117.

Fixes #26618

src/runtime/malloc.go

@@ -1012,7 +1012,7 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 	}
 
 	if rate := MemProfileRate; rate > 0 {
-		if size < uintptr(rate) && int32(size) < c.next_sample {
+		if rate != 1 && int32(size) < c.next_sample {
 			c.next_sample -= int32(size)
 		} else {
 			mp := acquirem()