[RISC-V] Assertion when using li with a symbol difference

[arichardson]

Assertion FixupKind != RISCV::fixup_riscv_invalid && "Unhandled expression!"'`

li t1, .Ltrap_vector_end - .Ltrap_vector

.global trap_vector
trap_vector:
.Ltrap_vector:
nop
nop
.global trap_vector_end
trap_vector_end:
.Ltrap_vector_end:

It appears this resulted in error: operand must be a constant 64-bit integer until recently: See https://godbolt.org/z/WKPEMsGTe

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

[asb]

CC @mtvec - I guess this behaviour changed with https://reviews.llvm.org/D135960

[mtvec]

Candidate patch: https://reviews.llvm.org/D158830

Note that the error message will be different with this patch (error: unsupported relocation type) but the assertion is gone.

[mtvec]

Cross-referencing another candidate patch: https://reviews.llvm.org/D157694