[AMDGPU] Handle mandatory literals in isOperandLegal

Extend SIInstrInfo::isOperandLegal to enforce a limit on the number of
literal operands for all VALU instructions, not just VOP3. In particular
it now handles VOP2 instructions with a mandatory literal operand like
V_FMAAK_F32.

Differential Revision: https://reviews.llvm.org/D126064