[InstCombine] Add test cases to show bad canonicalization of bitcasts (details)
[InstCombine] Add test cases to show bad canonicalization of bitcasts between x86_mmx and <1 x i64>. As the test cases show, we end up with an insert/extract and a bitcast to/from i64. x86_mmx is for some purposes conceptually a vector. We shouldn't be adding scalar conversions around it. Since _m64 is defined as <1 x i64> and intrinsics use x86_mmx as their input/output these extra scalar operations prevent the X86 backend from generating good code especially on 32-bit targets where i64 gets split.