[AMDGPU] Add verification for mandatory literals

Extend the literal operand checking in SIInstrInfo::verifyInstruction to
check VOP2 instructions like V_FMAAK_F32 which have a mandatory literal
operand. The rule is that src0 can also be a literal, but only if it is
the same literal value.

AMDGPUAsmParser::validateConstantBusLimitations already handles this
correctly.

Differential Revision: https://reviews.llvm.org/D126063