[MergeFuncs] Don't merge shufflevectors with different masks

When the shufflevector mask operand was converted into special
instruction data, the FunctionComparator was not updated to
account for this. As such, MergeFuncs will happily merge
shufflevectors with different masks.

This fixes https://bugs.llvm.org/show_bug.cgi?id=45773.

Differential Revision: https://reviews.llvm.org/D79261