[gvn] CSE gc.relocates based on meaning, not spelling

The last two operands to a gc.relocate represent indices into the associated gc.statepoint's gc bundle list. (Effectively, gc.relocates are projections from the gc.statepoints multiple return values.)

We can use this to recognize when two gc.relocates are equivalent (and can be CSEd), even when the indices are non-equal. This is particular useful when considering a chain of multiple statepoints as it lets us eliminate all duplicate gc.relocates in a single pass.

Differential Revision: https://reviews.llvm.org/D97974

(Note: Part of the reviewed change was split and landed as f352463a)