[InstCombine] Add test cases to show bad canonicalization of bitcasts between...

[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.