Synthesize RHS constants

[nunoplopes]

something like:

define i8 @src(i8 %y) {
  %x = sub i8 %y, %y
  ret i8 %x
}

define i8 @tgt(i8 %y) {
  %c = call i8 @llvm.symbolic.const()
  ret i8 %c
}

The goal is to synthesize a value for %c (0 in this case), or fail if no constant exists.

cc @regehr

[regehr]

one extra thing that would be excellent is an indication if the constant is unique, or perhaps an option to dump N constants instead of just the first one