[MIPS] Check if register is non-null before calling isSuperOrSubRegisterEq (NFCI)

D151036 adds an assertions that prohibits iterating over sub- and
super-registers of a null register. This is already the case when
iterating over register units of a null register, and worked by
accident for sub- and super-registers.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D151288

Author: s-barannikov

llvm/lib/Target/Mips/AsmParser/MipsAsmParser.cpp

@@ -3087,6 +3087,7 @@ bool MipsAsmParser::loadAndAddSymbolAddress(const MCExpr *SymExpr,
         MipsMCExpr::create(MipsMCExpr::MEK_HIGHER, SymExpr, getContext());
 
     bool RdRegIsRsReg =
+        UseSrcReg &&
         getContext().getRegisterInfo()->isSuperOrSubRegisterEq(DstReg, SrcReg);
 
     if (canUseATReg() && UseSrcReg && RdRegIsRsReg) {