compiler: refactor and add tests

This commit finally introduces unit tests for the compiler, to check
whether input Go code is converted to the expected output IR.

To make this necessary, a few refactors were needed. Hopefully these
refactors (to compile a program package by package instead of all at
once) will eventually become standard, so that packages can all be
compiled separate from each other and be cached between compiles.

compiler/testdata/pointer.go

@@ -0,0 +1,41 @@
+package main
+
+// This file tests various operations on pointers, such as pointer arithmetic
+// and dereferencing pointers.
+
+import "unsafe"
+
+// Dereference pointers.
+
+func pointerDerefZero(x *[0]int) [0]int {
+	return *x // This is a no-op, there is nothing to load.
+}
+
+// Unsafe pointer casts, they are sometimes a no-op.
+
+func pointerCastFromUnsafe(x unsafe.Pointer) *int {
+	return (*int)(x)
+}
+
+func pointerCastToUnsafe(x *int) unsafe.Pointer {
+	return unsafe.Pointer(x)
+}
+
+func pointerCastToUnsafeNoop(x *byte) unsafe.Pointer {
+	return unsafe.Pointer(x)
+}
+
+// The compiler has support for a few special cast+add patterns that are
+// transformed into a single GEP.
+
+func pointerUnsafeGEPFixedOffset(ptr *byte) *byte {
+	return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(ptr)) + 10))
+}
+
+func pointerUnsafeGEPByteOffset(ptr *byte, offset uintptr) *byte {
+	return (*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(ptr)) + offset))
+}
+
+func pointerUnsafeGEPIntOffset(ptr *int32, offset uintptr) *int32 {
+	return (*int32)(unsafe.Pointer(uintptr(unsafe.Pointer(ptr)) + offset*4))
+}