APX NDD instructions may be compressed when the result is also a source.
For 8/16b instructions, this may create partial register write hazards
if a previous super-register def is within the partial reg update
clearance, or incorrect code if the super-register is not dead.
This change prevents compression when the super-register is marked as an
implicit define, which the virtual rewriter already adds in the case
where a subregister is defined but the super-register is not dead.
The BreakFalseDeps interface is also updated to add implicit
super-register defs for NDD instructions that would incur partial-write
stalls if compressed to legacy ops.
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