This commit fixes pr31143 (https://llvm.org/bugs/show_bug.cgi?id=31143).
ExecutionDepsFix pass couldn't identify the partial register update of ROUNDSSr:
%XMM1<def,tied1> = ROUNDSSr %XMM1<undef,tied0>, %XMM0, 9
The root cause of this was because ROUNDSSr instruction is marked as reading %XMM1, but it's actually reading an undef value.
This patch fixes MachineInstruction - now a use of an undef value isn't considered a real use anymore.