Pointer-to-local variable probably doesn't follow correct semantics

[litherum]

Migrated from https://bugs.webkit.org/show_bug.cgi?id=179245

Imagine the following

thread int* foo(int x) {
int y = x;
return &y;
}

fragment void bar() {
thread int* z = foo(6);
*z = 7;
foo(8);
// Now, *z should equal 8.
}

Our instantiation will currently create a copy of foo() at the two call sites, thereby leading to two distinct "y" variables. This probably means the above doesn't lead to the correct result.