[llvm-mca] Add tests for instructions that implicitly clear the upper portion of a super-register.

On x86-64, a write to register EAX implicitly clears the upper half or RAX.
128-bit AVX instructions clear the upper 128-bit of the YMM register that
aliases the XMM definition register.

llvm-mca doesn't know about register writes that implicitly clear the upper
portion of an aliasing super-register. This issue will be fixed in a future patch.

llvm-svn: 334742