[NFC] Fix possibly deref nullptr

1. In X86LowerAMXType.cpp dyn_cast could lead to UserI be nullptr which coud be dref in IRBuilder constructor.
2. In AsmPrinter.cpp, doInitialization could make MMI be nullptr if MMIWP->getMMI() is false, then the deref after could be unexpected.

Reviewed By: skan

Differential Revision: https://reviews.llvm.org/D157948