[RuntimeDyldChecker] Support offset in decode_operand expr

In RISCV's relocations, some relocations are comprised of two relocation types. For example, R_RISCV_PCREL_HI20 and R_RISCV_PCREL_LO12_I compose a PC relative relocation. In general the compiler will set a label in the position of R_RISCV_PCREL_HI20. So, to test the R_RISCV_PCREL_LO12_I relocation, we need decode instruction at position of the label points to R_RISCV_PCREL_HI20 plus 4 (the size of a riscv non-compress instruction).

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D105528