Mutable variables alias instead of copy

[jfecher]

x = mut 4
y = x
print y
x := 5
print y

Expected to print 4 twice, but it instead prints 4 then 5.
This can be seen in the hir (--emit-hir):

x_v0 = (#StackAlloc 4_i32);
y_v1 = x_v0;
(print_v2 y_v1);
x_v0 := 5_i32;
(print_v2 y_v1)

x is a stack-allocated pointer, but y does not dereference it as it should. This is a code-gen issue.