[X86][MC] Avoid emitting incorrect warning for complex FMUL

We will insert a new operand which is identical to the Dest for complex
FMUL with a mask. https://godbolt.org/z/eTEdnYv3q

Complex FMA and FMUL with maskz don't have this problem.

Reviewed By: LuoYuanke, skan

Differential Revision: https://reviews.llvm.org/D130638