runtime (gc): work around IR generation bug with right shifts

Right shifting more than the size of a type is considered legal by the Go spec, but not by LLVM.
As a result, alignMask was returning poison values on 16-bit and 32-bit systems.
This is a temporary workaround that assumes that the input size is small.

Author: niaow

src/runtime/gc_conservative.go

@@ -242,8 +242,11 @@ func alignMask(size uintptr) uintptr {
 	x = x | (x >> 2)
 	x = x | (x >> 4)
 	x = x | (x >> 8)
-	x = x | (x >> 16)
-	x = x | (x >> 32)
+
+	// These do not work due to bugs in our codegen for right shifts.
+	// These are not strictly necessary for this use case, as the input is known to be < 256.
+	//x = x | (x >> 16)
+	//x = x | (x >> 32)
 
 	// The highest bit's value is the alignment.
 	// Shifting down gives us a mask that is one less than the alignment.