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InstCombine: Fold comparisons between unguessable allocas and other pointers

Description

InstCombine: Fold comparisons between unguessable allocas and other pointers

This will allow us to optimize code such as:

int f(int *p) {
  int x;
  return p == &x;
}

as well as:

int *allocate(void);
int f() {
  int x;
  int *p = allocate();
  return p == &x;
}

The folding can only be done under certain circumstances. Even though p and &x
cannot alias, the comparison must still return true if the pointer
representations are equal. If a user successfully generates a p that's a
correct guess for &x, comparison should return true even though p is an invalid
pointer.

This patch argues that if the address of the alloca isn't observable outside the
function, the function can act as-if the address is impossible to guess from the
outside. The tricky part is keeping the act consistent: if we fold p == &x to
false in one place, we must make sure to fold any other comparisons based on
those pointers similarly. To ensure that, we only fold when &x is involved
exactly once in comparison instructions.

Differential Revision: http://reviews.llvm.org/D13358