[llvm] [AMDGPU] Use `S_BFE_U64` for uniform i1 ext (PR #69703)

[Pierre van Houtryve via llvm-commits]

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@@ -29,10 +29,11 @@ define amdgpu_kernel void @saddo_i64_zext(ptr addrspace(1) %out, i64 %a, i64 %b)
 ; SI-NEXT:    s_mov_b32 s0, s4
 ; SI-NEXT:    s_mov_b32 s1, s5
 ; SI-NEXT:    s_xor_b64 s[4:5], s[6:7], vcc
-; SI-NEXT:    v_cndmask_b32_e64 v0, 0, 1, s[4:5]
-; SI-NEXT:    v_mov_b32_e32 v1, s11
-; SI-NEXT:    v_add_i32_e32 v0, vcc, s10, v0
-; SI-NEXT:    v_addc_u32_e32 v1, vcc, 0, v1, vcc
+; SI-NEXT:    s_bfe_u64 s[4:5], s[4:5], 0x10000
+; SI-NEXT:    s_add_u32 s4, s10, s4
+; SI-NEXT:    s_addc_u32 s5, s11, s5
+; SI-NEXT:    v_mov_b32_e32 v0, s4
+; SI-NEXT:    v_mov_b32_e32 v1, s5
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Pierre-vh wrote:

There is no divergence analysis bug, it's just that we don't have i64 scalar compares.
Would it be correct to mark a `setcc` as a source of divergence if we know it has to lower to a v_cmp?
The i1 it returns is no longer uniform, it depends on exec, right?

https://github.com/llvm/llvm-project/pull/69703