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When we find something like this: ``` %a:_(s32) = G_SOMETHING ... ... %select:_(s32) = G_SELECT %cond(s1), %a, %a ``` We can remove the select and just replace it entirely with `%a` because it's always going to result in `%a`. Same if we have ``` %select:_(s32) = G_SELECT %cond(s1), %a, %b ``` where we can deduce that `%a == %b`. This implements the following cases: - `%select:_(s32) = G_SELECT %cond(s1), %a, %a` -> `%a` - `%select:_(s32) = G_SELECT %cond(s1), %a, %some_copy_from_a` -> `%a` - `%select:_(s32) = G_SELECT %cond(s1), %a, %b` -> `%a` when `%a` and `%b` are defined by identical instructions This gives a few minor code size improvements on CTMark at -O3 for AArch64. Differential Revision: https://reviews.llvm.org/D76523
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