cmd/compile: -d=checkptr doesn't work with named unsafe.Pointer types

[mdempsky]

This package fails to compile with -d=checkptr:

package p

import "unsafe"

type P unsafe.Pointer

func f(p P) *int { return (*int)(p) }
func g(p P) P    { return P(uintptr(p) + 1) }

This is because the runtime functions have unsafe.Pointer parameters, but currently when inserting instrumentation, we pass the unsafe pointers as-is (i.e., as P-typed above, which isn't assignable to unsafe.Pointer).

Fix is to just add appropriate explicit conversions in the instrumentation.

Reported by Asif Jalil on golang-dev.

[gopherbot]

Change https://golang.org/cl/201782 mentions this issue: cmd/compile: fix -d=checkptr for named unsafe.Pointer types