AArch64: Don't modify other modules in AArch64PromoteConstant
Avoid modifying other modules in AArch64PromoteConstant when the
constant is ConstantData (a horrible accident, I'm sure, caught by an
experimental follow-up to r261464).
Previously, this walked through all the users of a constant, but that
reaches into other modules when the constant doesn't depend transitively
on a GlobalValue! Since we're walking instructions anyway, just
modify the instructions we actually see.
As a drive-by, instead of storing Use and getting the instructions
again via Use::getUser() (which is not a constantant time lookup),
store std::pair<Instruction, unsigned>. Besides being cheaper, this
makes it easier to drop use-lists form ConstantData in the future.
(I threw this in because I was touching all the code anyway.)
Because the patch completely changes the traversal logic, it looks
like a rewrite of the pass, but the core logic is all the same (or
should be, minus the out-of-module changes). In other words, there
should be NFC as long as the LLVMContext only has a single Module.
I didn't think of a good way to test this, but I hope to submit a patch
eventually that makes walking these use-lists illegal/impossible.