Provide input to LLVM's TBAA (type-based alias analysis)

[JohanEngelen]

Currently we cannot optimize this code further:

int foo(int *x, float *y) {
  *x = 5;
  *y = 8.0f; // cannot alias with x because of types
  return *x; // here we reload from memory, instead of returning 5
}

Optimization is continued by Clang for C++ (i.e. returning 5 as immediate operand) using TBAA.
https://lists.llvm.org/pipermail/llvm-dev/2018-July/124493.html

This needs thorough checking: the notion that pointers of different types cannot alias must be added to the spec if it is not already in the spec. (note: int[] and int* and int[5]* can alias and can also partially alias)

This is not a minor task.

[JohanEngelen]

Concerns: https://blog.regehr.org/archives/1307

Perhaps only enable with an explicit fstrict-alias compile flag.